I have not bought a bike off of ebay, but have gotten several through Craigslist. Or looking around the house, um many from Craigslist.

The tally:
2 bikes that I love that were not deals: bikes 1 and 2.
1) An orange Schwinn Fastback Stingray with all original parts, great remaining paint job... still $200 is a little high for a 1972 coaster Stingray. This was a (considered / researched overnight) impulse buy while getting my mom a cruiser bike.
2) My baby, a 1998 lugged steel celeste Bianchi Eros. $580 is definitely on the high side even considering great shape / low milage, but I love her and don't regret a penny. I was confident buying a Bianchi road bike for fit, because I have a 2005 Bianchi Castro Valley commuter. I also brought my trainer to set up the bike and for initial test of comfort before test riding.

1 bike I overpaid for and regretted... bike 3.
1) This was from college kids who "refurbished" bikes... the one that I wanted to look at was sold, and there was a Mixte -- really was an impulse buy, $120 for a 1970's Raleigh Sprint 5. This is about average for a tuned-up bike, but this one was not done very well. I've re-tuned but don't like the upright ride... will offer it to friends first, then re-Craigslist.

2 great deals that I am thrilled with, bikes 4 and 5
1) Bike for my sister: 1970's Seinke Mixte, 45 cm; new 24" tires, newish brakes and levers, newish click shifter, and new pedals. Only $75 -- wonderful find from a person that just likes scrounging parts, building up bikes, and selling them near cost. (Versus most people that shine up old bikes with minimal tuneups and sell for 2-3x original cost.)
2) Mixte for me: early 1980's Raleigh Super Course Mixte, with 531 Reynolds chromoly, including new 700c wheels with a 16 cog. The front has a 40 chainring, and I can take the upper 52 chainring off; I just need to slap on tubes, tires, and a chain and I have a $95 "Fixte." The frame does to be redone, great for me: I want a pixte (pink fixie mixte). I couldn't ride on rims(!) but I did measure the dimensions.