I've been fortunate to have had good luck purchasing from Craigslist.

I bought our EZ Tandem from a local Craigslist ad...a really fun bike in great condition for a fair price.
I also bought my Volae Tour from a Craigslist ad from downstate, about 3 hrs. away. This bike was being offered by a very active participant on BROL though I was unaware of who he was until much later, but I got a quality machine at a very good price.

I just recently purchased an English 3 spd. and a B.O.B. Yak from local Craigslisters, again honestly represented and priced fairly.

Like any other used marketplace, you've got to know your equipment and it's value and ask the right questions. There are some diamonds among the stones out there.