One of the tricky parts of navigating the Net is understanding how to narrow your search. World Wide Arts Resources offers an entire section on antiques, including sites specializing in architectural inventory. Better yet, use a metasearch engine, which searches several ways at once. Be as precise as possible in conveying information to the search engine. The more specific you make your search, the easier it will be to home in on a given item.
Some dealers accept returns.
Others have a buyer-beware policy. As the buyer, you send your check to the escrow service. They'll hold it until you've had a chance to inspect your purchase. If all is well, they'll release the money to the dealer. If you don't like the item, ship it back.