I also sell car parts. I liquidate large salvage yards and have employees that strip down cars and list parts on eBay and enter them in to my AS400 inventory system which is linked to my eBay account(s).
There are 12-20 car parts per car that sell very easily on eBay. I'm not going to tell you what these parts are, because it took me a long time to figure out what they were. These are items that can be listed competitively and be sold within 7-14 days. The average cost of a listing for a non-TRS is .50 - this is a lot of money if you are listing 1000 car parts at a time. Most other car parts can be listed competitively and take 4-8 weeks to sell. This also depends on the make and model of the car. With Google sending next to no traffic to eBay anymore, this is probably going to take a lot longer for parts that aren't in high demand of needing to be replaced.
If you can, try to become a TRS before you start listing items. The 40% off in fee's saves a lot of money. Also use PayPal Micropayments for your parts that cost less than $10.00 - You simply need to sell about 100 items a month for 3 months, and you have to ship out fast. You better be printing out your shipping labels in less than 12 hours of the time of the purchase. Contact the seller of items from Hong Kong that sell on eBay Motors and buy 100 of something such as
http://www.ebay.com/itm/1-pcs-new-U...=&orig_cvip=true&rt=nc&_trksid=p2047675.l2557 - You will become a TRS - just ask to purchase 100 of these and ask for a discount, and 100 of another similar item, and another 100 of a similar item. You should be able to unload something so small and cheap and commonly purchased very quickly.
Then take advantage of things like "100 free listings this month!" promotions which come out every month, and list all of your tier 2 items such as things like center consoles which typically have a good profit margin but do not sell often.
When things pick up and you begin to profit, you can move to selling your tier 3 items which are items that get very little views and not commonly purchased, but when they are, offer a really good profit margin. For me, these are things like switches and dash pieces, airbag control units, etc.