The best places to sell first day covers is at a local dealer, which can be located by perusing the phone book for "stamps for collectors" or visiting collector societies to find dealership databases. Other avenues exist as well, such as pawn shops or online auctions, but the best way to sell valuable collectibles safely is to go through a face-to-face transaction, explains the American First Day Cover Society.
To get a rough idea of the value of a first day cover prior to visiting a dealer, consult books with value estimates to give you a starting point. Be aware that this is typically the price a dealer sells the item for, so generally you receive under that amount, unless you directly sell it to another collector by posting it on websites, such as Craigslist or Amazon.
An additional way to sell first day covers is to go through an online auctioneer or auction website, such as eBay. If you live in an area without strong demand for these collectibles, selling on the Internet might be your best bet, as this allows you to reach everyone across the globe with access to the Internet, tremendously increasing the number of potential buyers.