Are you looking for NOS parts or just clean used? NOS is likely to come with packaging from a dealer with a brick-and-mortar shop. You can always email if you need to know. Even from a private seller, the odds are low you will get stolen merchandise, especially if it comes with any kind of documentation.

I don't think there is any way to guarantee you won't buy a stolen article, but organizing an ebay auction seems a poor way to unload bits and pieces for a petty thief. It's time consuming. A street thief would need to be patient and disciplined for that.

I would guess that most sellers are parting out a bike and recouping some money for buying new gear. Some may be speculators that buy nice bikes whole in newspapers and through Craigslist and part them out for fun and maybe to make a little money. It would seem hardly worth the trouble, though.