Firstly let me say this: what you're experiencing just comes with the territory. any form of retail sales will experience scammers and consumer bullies that are after free or nearly free stuff. I've been ebaying for half a decade and luckily have not had to deal with them often, but i've had my experiences.
Your ability to defeat these people will depend on a few factors, including age of account, volume of (positive) sales, revenue generated, level of activity, etc. The older, bigger, and more reliable you are the nicer ebay will play with you. If you're a new account with low activity you will probably just have to eat this until you grow a bit more.
Be aware - when a buyer claims what they received is not the same as what the listing claimed (is not in same condition, completely wrong item, or "empty box"), ebay ToS instantly sides with the buyer, and they will tell you that. it's a crappy situation but it's what they do in order to maintain a "happy, guaranteed safe buying community" to protect buyers from shady sellers. This means any attempt at winning cases like this is an uphill battle every time. it IS however possible to come out ahead.
NOTE: Ive never dealt with two scammers at once, this is going to look bad for you, I would try to segregate them and deal with them separately, potentially ignoring one and not bringing any fuss to it if possible until after the first is resolved, at this point that might not be an option though. I will discuss the steps to take as if we are dealing with one single issue, but the same thing applies to both problems.
what you need to do:
1. call ebay and get to their native english speaking reps at all costs.
When you get out of the que if they have asian/indian accents, either hang up, or make up an excuse to get xfered, once xfered, you can either try working with whatever department you moved to or request a transfer back if you know they arent where you need to be. repeat this process until you get to a native english speaker, they're typically american or irish.
Their indian/asian reps are all but useless, I think its a language barrier thing. They don't speak fluent english and thus can't really grasp the details of what's going on with your situation, and thus just stick to their script without deviating. I've had some success with their Philippians girls but its been limited, so I just avoid it all together.
You may have to wait and call at different times depending on your location, I shoot for mid day (US)
2. Social Engineering is the best hacking method.
explain the situation CALMLY AND FRIENDLY. Have the mentality that you and ebay are friends, you guys like each other and get along well, and you need your buddies help to fix this issue and defeat evil. It is not the reps fault for what's going on and you know that, it's just upsetting. (I make sure to verbally express this if I start ranting or begin getting worked up). The goal is to make them like you and be on your side. If you can do that you have a MUCH higher chance of a positive outcome. Once I was SOL on a scamming situation and got the phillipean girl to acknowledge "Sir please understand this is not my decision, despite what i think and who I agree with I have to follow ebays procedures as it's not my decision to make." This essentially made her side with me, and in turn she subliminally fought to help me, then it's just a matter of holding out long enough on the exchange for her to figure out how to do it. I eventually walked away with my problem solved, probably took 30-45 mins in total.
Depending on your accounts stats, the scammers stats, and the amount of $ in question, and the details of your case, they may side with you and close the issue in your favor, give you advice on how to make it turn into your favor, or grant ebay coverage (ebay refunds the buyer out of pocket, they keep the item or "empty box", and let you keep your sale). be aware the last option does not protect your feedback most of the time.
3. if #2 fails and you cannot get them to budge, request to speak with a supervisor.
eBay reps will put you on hold for no longer than 2 minutes, I'm 99% sure they have a 2 minute tops holding time rule as every 2 mins they will come back to assure you they haven't forgotten about you and ask you to wait another 2 mins. They may try to wave off your request initially reaffirming that the decision theyve made is law and the supervisor will reiterate it, don't deviate. They may come back and tell you "My supervisor is busy sir but if you could call back later...", dont deviate. They may put you on hold for 10-20 minutes, dont deviate. They will NEVER terminate the call (less breaking rules and drop you) or deny your request to speak to a sup in the end. You will always be allowed to speak to someone as long as you're willing to hold. Remember step number 2. stay calm, stay friendly "no problem I'm fine waiting" etc.
you -WILL- speak to the supervisor, at that point it should be a native english speaker, repeat the previous steps with them and remember - BE THEIR FRIEND. theyer a supervisor for ebay, you are a seller for ebay. You pay their paycheck, you two are essentially work colleagues and like each other. build raport where you can, if you notice an accent ask questions about it, chit chat and be friendly during downtime in the call. The supervisors have the power to make all your problems go away at the push of a button.
Just as I mentioned with the basic reps, I will copy paste from there:
Depending on your accounts stats, the scammers stats, and the amount of $ in question, and the details of your case, they may side with you and close the issue in your favor, give you advice on how to make it turn into your favor, or grant ebay coverage (ebay refunds the buyer out of pocket, they keep the item or "empty box", and let you keep your sale). be aware the last option does not protect your feedback most of the time.
The difference is basic reps will normally grant ebay coverage for transactions under X amount ( I think 20 bucks) as long as you're a good looking account, and don't really care about the details of your individual situation.
The supervisors however will read the situation in detail, look at messages, transactions, tracking numbers, addresses, peek at the scammers acc for other complaints, etc. They will also listen to you explain your entire side of the story and decide how to handle it. They will MUCH MORE OFTEN hear you out and if you are truly the good guy and evidence from previous transactions shows you're a good, friendly and positive seller and dont go through this too often, they'll just cover you to keep you a happy seller*
*I get treated -very- good by sups when I speak with them, this may be due to the level and quality of my seller account. I assume the bigger and better you are the more leniency they give during issues.
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Those are the concrete steps to follow, A few other thoughts specific to your current situation:
With the one where the guy that sent a return to a random business in the same zipcode, try to get some form of documentation of the address delivered from USPS, if you can get an email or reference code or something to verify that and show it to a supervisor, maybe ask to email it to them or something, I would expect that to change course from bad to good results for you. supervisors can function outside of the script well and give human interpretation to a situation, this very well may be your best life line.
With the guy who claimed you sent an empty box, you might be able to speak to USPS and verify the original weight of the package. If you created tracking they had to weigh it to apply proper costs, it may be arguable "The box was 2 pounds when I shipped it, do you have any idea how big an empty box has to be to reach 2 pounds?", again, this might be best suited for supervisor discussion.
Any form of proof that the buyer is lying will be the turning point for you, either making ebay side in your favor, or agreeing to cover you cause its obvious they're scamming but using loopholes and thus cant be stopped.
I hope this information helps you, like I said you're in a tough spot (especially with 2 at once) and you might just have to eat the costs for these ones, but the above information is going to be your best shot. This wont be your last time experiencing the issue so if nothing else it's a good training ground.
I can't stress enough to be friendly and build rapport with who you work with, making them like you is the key to a positive result for you.
If you have any questions feel free to ask. Also please note I have opened a thread for general ebay advice/solutions/discussion as I am interested in offering a service. I know I need to prove myself and become more active, so feel free to go there for anything you need help with.
*edit* Added link to thread:
http://www.blackhatworld.com/seo/got-questions-about-ebay-ask-here.883878/
Also if this was helpful to you or others, giving thanks would be appreciated