Most of the "good" bikes on CL are stolen.... it's very obvious most of the time. Usually they're listed without much information, or something ubiquitous like "bought new last year, rode twice, didn't like it" on a $1500 bike they're listing for $350, that has obviously seen more usage (and upgrades) than that. Some people just plain can't tell you ANY info about the bike either, like frame size or year, etc... that's a huge red flag IMO. Anyone who knows what they are looking for can quickly figure both out.

I personally, wouldn't buy a late-model bike without an original sales receipt that can be confirmed by the shop it was purchased from.... I felt iffy buying my mixte off CL, honestly, and that bike is 24 years old.. probably not stolen, but I don't know for sure.