Oh yea, forgot to mention that eBay used to take a much smaller bite out of your selling price back then.
I think it's 9% now.
It's actually 12% or so.
Geez, some of you guys must be rich!! Some of you are talking like $100 profit for doing almost nothing isn't worth it. Geez...that's two full days of work for some people.
I've been flipping guitars/basses/amps for almost two years. Right now, it is about my sole source of income. It started with one 1978 P-bass that I bought for $650 from a pawn shop on ebay and resold for $1000 on ebay two days after receiving it. He claimed there was a piece broken on it (there was a bridge cover on it) but I went on the hunch there was nothing wrong and it turned out all it was was the bridge saddle rattling down there

(the G string was missing). Anyone looking for a bass for their hands would have avoided that...but for a flipper, ignorance is a boon. That was when I realized the money was there. I have bought roughly 300 guitars and basses in the last two years - mostly on ebay and probably 25 on craigslist.
There are dozens of factors when doing it full-time. I can't even begin to go into detail about it. It takes a long time to learn the ropes in and out. Above all, you need to have the time to do it. The deals pop up and they go fast; any great deals on craigslist can be gone within a matter of an hour and ebay deals go within minutes. I spend lots of hours surfing and checking repeatedly (although...I'd rather waste time doing that then waste time fighting dragons on some online game for 8 hours a day!)
Generally, my sales only generate an average of about $75 each after all applicable deductions. The main way to maximize profit in an ebay setting or even craigslist is parting them out. Just like cars, the parts are worth more than the whole..ESPECIALLY the vintage. I can buy Musicmaster basses that are battered and bruised for $250 and resell the parts for $400-$450. Musicmasters are my favorite because they are cheap but their parts are very valued because it's not easy to find a single part on an old bass. Instruments not described correctly are the next best profit-house. People are scared to buy from pawn shops but I could care less; I just part it out.
I only dabble about exclusively in Fenders because I like them (keen interest is the first way to making a business out of something) and I can part them out. I'll also mess with vintage Fender amps, Ampeg, and Ric. If you get a clean system of how to do it just about anything can make you money. Right now since I'm jobless I have started buying things that will make me less a profit ($50-$60 each guitar) but at the end of the day just one of those is more than the average Wal-Mart employee makes in a day. It's fun, interesting...and I can get my own gear and sweet stuff for my friends too. And I get to try out just about every model of Fender under the sun.