First, an antique firearm is defined by law. If the firearm was made PRIOR to the year 1899, it is not a firearm, but an antique firearm. As such, it falls outside the 1968 Gun Control Act. By FEDERAL law, it can be shipped directly to the buyer (State laws vary, especially NY, NJ, CA) and CAN be sent by US Mail.
For future readers- this does not mean that the MODEL of the gun was made prior to 1899, but that the specific gun in question was made before 1899.
Other antique firearms include those that can only be loaded with loose ball and powder (muzzle loader, ball and cap, etc) no matter how old.
Modern REPRODUCTIONS of antique cartridge arms are not antiques if they are made in a cartridge that is available in normal business and trade.
Regarding your auction- GOOD pictures, GOOD descriptions. The guy complaining about a M-N buttplate sounds like a PITA. But do not surprise bidders. Collectors of antiques generally have different expectations than someone looking to buy their first rifle. Show the warts- good photos, good detail, lighting, in focus.