I've bought 2 bikes off ebay, and a bike frame - one bike & a frame off craigslist. The bf's bought 1 bike off ebay and 2 from craigslist.

Craigslist is easy - you can see the bike and you can test ride it. Depending on your area, sometimes it can be pretty competitive to get the really good deals on nice bikes... At least it is around here. And there's a heck of a lot of people that think their 10 year old schwinn or some such is still worth $200. But the bikes we've bought on craigslist have been great deals.

As for ebay - definitely pay a lot of attention to the seller's measurements. I once bought a frame that I know & wanted in a 49 cm size... It was listed as a 49 cm. The frame arrived and it had a sticker on it that said 52 cm frame. Making it way too large for me, but it fit the bf and he built it up for himself. The seller did reluctantly state that he would take that frame back & refund my money if I really really wanted.

Another bike was significantly upgraded from the original specs - I think I eventually found that the cassette was not ultegra level like was claimed in the ebay listing (not a huge deal, but not something I noticed till I changed the cassette for some reason)... and someone had put 172.5 mm cranks on a 650c 49 cm bike. I hadn't thought to ask them the crank length - and found it bizarre that anyone would think that someone needed 172.5 mm cranks on that small of a bike anyways - I bought smaller cranks to put on it, eventually decided I hated the bike and ebayed it - I didn't lose any money on the whole deal.

My litespeed I got on ebay for an amazing price, was in great shape - at the point I saw it, it kinda had no details other than 49 cm litespeed and some pics. I went into the archives on litespeed, figured out what it was & what the components were on it if it was still stock, and just did the buy it now because what the seller wanted was at a price that I was willing to deal with what I got. I wasn't too clear on the sizing, but if it didn't fit, it just meant the bf was going to get a great price on a litespeed and I was gonna pout. But it worked out and it's my favorite bike ever.

My bf's got his litespeed on ebay. A good price, but not great - except that I guess DHL must have laid the box flat in the truck & packed a bunch of boxes on top of it and the box musta slide back and forth in the truck - so while the bike was absolutely fine, it had screwed up the hub of the wheel. DHL gave him like $800 for a new wheel in the insurance claim... thus making it a great deal.

bikepedia.com is a good resource when you're trying to figure out what a buyer is selling and a price point.