It may not be stolen bikes. Some people will just not know what you are talking about and equal that with you asking for private information. Sellers that don't provide enough information, also are too lazy to look up the SN.
I don't buy used bikes. But once I helped my MIL to buy a car in the up to $5K range. I assume none of the cars were stolen. But the responses I got when I just asked some reasonable questions. Like if the timing belt maintenance was done, of if they have the title (i.e. paid off). Some ignored me, some just told me to "f... off!". Some cars turned out to not have registration anymore and I was told I could do a test drive on a road nearby that "for sure never has cops". i just translate that experience into bike sellers.