How do you tell an honest private seller and actual owner from a reseller?
I really do not want to end up in a situation where someone sells me a problematic car pretending it is from a single owner. Is there any way to tell right away if it is a reseller or just a regular seller and former owner?

Even an honest seller can leave something out, not out of bad intent, but simply because they do not know. Or because they want to sell it for more. So relying on the seller’s words alone is a bad idea anyway.

I think you can tell a private seller from a reseller by several signs taken together. A reseller usually does not know the ownership history well, cannot clearly say where and how the car was serviced, and often tries to steer the conversation away from specific questions. Checking the VIN through FAXVIN helps you quickly see how many owners the car had, and you can then ask the seller about it and compare the info. But in any case, every seller needs to be checked and should never get 100 percent trust right away.