I posted this in the cheap hobbies thread, but I have 2 friends who have evolved Ebay into a real money maker. One sells used tools (quailty names and vintage) and militaria and the other does vintage clothing, specailizing in vintage denim.
They both make big $$$. One cleared over 35k last year working 4 days a week, and only 5-7 hours those days. They get all the items they sell from flea markets, yard sales, thrift shops, and pawn shops.
Its easy enough. Pick a subject that will sell halfway decent and that you know about or have an interest in, then look back at all the past auctions and research what sells and what techniques work. the key is never paying much for anything, so if it doesn't sell your not out anything. My friend that does vintage clothing finds used jeans for $1-5 and gets $50-300 or even more from them, usually to buyers in asia.