First are you having it shipped, or is it local?
If shipped, here is how it’ll work. You paid the cellar with a money order so they will have their money. Firearm ships out to an FFL dealer, and they log the gun in as part of their inventory. If you don’t pass your background check, the dealer has your gun, and the seller has your money… And you have nothing.
If it’s local multiple do money order or cash, and no personal checks