Ask BEFORE you buy.
It doesn't do much good to ask "did I get a good deal?", "Is this a good bike?" or "Can I convert this?" after you've already bought said item. If the internets says, "No. You got screwed", what are you gonna do now?
- Craigslist can sometimes be a good place for deals. But, often it's the place where people unload stuff that isn't worth posting to ebay.
- The single speed / fixed / track market is a pretty mature these days. All of the garages, basements, and thrift stores have been picked dry. There aren't that many amazing finds, it seems.
- Lots of people know of the fad so the prices are either OK, high, or super-fanboy-inflated. The OMG-look-what-I-got-for-hella-cheap deals are far and few between.
So...if someone posts a bike for $X on CL. It's probably worth less than that. Also watch out for "vintage frame that is just PERFECT for a conversion to a fixie!!!!" And if you care (or dare) enough to ask what we think, ask before you spend the money. It sort of works out better that way. And while you are searching, read the late Sheldon Brown's website to learn about what you are getting into to see if you are up for it: http://sheldonbrown.com/fixed-conversion.html