I think anytime you have that many 'pre-owned' bikes you run the risk that some of them are stolen, regardless of how the current owner acquired them.

Some of the bikes listed on Craigslist are obviously stolen - like the guy who used to post from a Times Square internet cafe saying "bring cash - ride home tonight." Some are probably perfectly legitimate. Either way you should get a receipt just in case you ever need to prove you paid for it.