A red flag should go up. Unscrupulous sellers prey on the greedy or the uninformed. Of course the buyer envisions the end of the nineteenth and beginning of the twentieth centuries. Take your time. Buying on impulse, buying the deal that is too good to be true and won't be there tomorrow is a recipe for disappointment.
Auction houses show items to be sold before the auction begins.
Get there in time to take advantage of that. Online auctions are typically active for a week. Use that time to carefully identify the item and its value. If you sincerely express your interest, dealers and brokers will often give you a brief time to consider your purchase before they sell it to someone else. Most importantly, the seller should provide an accurate and detailed bill of sale stating that the item is just what you agreed to buy.