Heya, guys. This is my first post here, but I couldn't help but give my advice on this subject.
A few years back, my best friend and I were selling digital products. We didn't have problems at first because most transactions were around $10. After a while though, we were doing deals for a few hundred (which was huge for that niche). We ran into our firs "unauthorized transaction" issue and then got smart from there. Here is what we did and *never* lost another penny:
First, we would never accept payment that wasn't classified as "Good: Non-Auction" (or whatever it's called). This protected us from the liability of providing a service. We would be up front with people about why we required that, and the people who were doing business legit had no problem with it.
Next, we required accounts have confirmed shipped addresses.
After any major transaction, we would mail a letter (with tracking of course) to the confirmed address pretty much just summarizing the transction. If there was ever a person trying to get a chargeback, we would just submit the proof to PayPal that the "Goods:Non-Auction" were shipped, tracked, and delivered to the confirmed address. Make note that transactions over a certain ammount ($500 I think) need to be mailed via Certified Mail for PayPal to protect the seller.
The best time ever was when a kid took his parents credit card to buy $800 worth of stuff and then try to get their money back. Needless to say, we won that dispute, and the parents probably beat the life out of him.
Anyways, I hope this post helped. It was a bit to write from my iPhone, but it is a topic I feel strongly about. If PayPal didn't have the market monopolized, I'd stay as far away from them as possible.