People can make a good living selling used auto parts online, specially on websites like ebay, amazon, or having you own store.

What I’ve learn through the years is that selling parts is not the issue, the issue comes with shipping cost, the weigh of the parts and the distance to ship will influence how much you make, there is also fixed shipping cost with USPS, UPS and DHL.

How much money can you make? This depends on your efforts, the more you sell the more you make, so listing more products online will give you the more chances to sell, also pricing will affect if you sell or not. What we do is look at the price for the same part from our competition and then reduce our price by 10 cents, 50 cents or even 1 dollar, people will buy yours if the price is less by just a few cents, always offer free shipping and give people a 30 days guarantee on everything (at leats).

Here is a great article that explains it very well,