They will send you a FFl. Do not sign the original (save that to make photocopies). Make a few copies and sign those. Mail or bring a signed copy to the seller and that is it. You must also start a bound ledger book to record your C&R purchases. Must list from whom and when you purchased it, serial numbers and then the date and to whom you sold it to.
Remember that as a C&R you are a collector and not a gun dealer so you have to limit what you sell out of your collection.