If you buy something with a C&R FFL, what sort of records is the seller required to keep on said transaction?
You need to log the firearm's serial number, seller, importer (if of foreign manufacture), caliber and type (rifle or handgun) into your C&R log within 24 hours of receiving the firearm from your shipper. That's it.
At some point, you may have to show your C&R firearms and log book to an ATF agent - The ATF seems to have quite a bit of extra time on its hands right now, so "audits" of C&R collectors are not unheard of. As someone who has been recently "audited", I can assure you that it's no big deal.