R. A. Siegel Auction Galleries

This auction house site offers an extensive online database of US stamp information. Includes PDF catalogs of past Siegel auctions, an encyclopedia of 19th-century issues (such as the 1845 New York Postmaster’s Provisional and the 1851 One-Cent Franklin), prices realized for various rare stamps, and images of a variety of interesting philatelic materials. A great deep dive on US stamp collecting!