I don't know if this will help you out with the seller, but about the only comprehensive listing of antique firearms was published by James Wesley, Rawles, The Pre-1899 Antique Guns FAQ. The gun list is alphabetized at the end of the FAQ and you can find the S&W section.
As Ed states the same thing as the FAQ that all single action S&W top break revolvers and Model of 1891s are considered antiques. I have never found a BATF list of antique firearms, so assume they leave it up to the seller to make the determination if their gun was made before 1899.
The Pre-1899 Antique Guns FAQ