Selling stamps used to be real easy, when just about every town had one or more stamp stores that a person could visit. Today, stamp stores are pretty much a thing of the past.
There are still stamp dealers all across the U.S., but they are generally "private" dealers, that buy and sell stamps by appointment or through other means, such as shows and bourses. We used to call them "vest-pocket" dealers.
If you want to sell your stamps to a dealer, the first place to check is your local phone directory "yellow pages", under the category "stamps". There are usually a number of private dealers listed, with phone numbers, and sometimes even a stamp store! Just call one or more of them and setup an appointment to have them look at your collection. If you don't really know the person though, I would recommend meeting them at a neutral site, at least for the first visit, maybe a library or hotel lounge close to you.
Many larger towns have clubs or dealer organizations that put on stamp bourses occasionally. This is also a good avenue, in which to talk to one or more dealers about selling your stamps.
The A.P.S. also has a directory of affiliated stamp dealers on their site. You can click on this LINK TO APS STAMP DEALERS , to see if there are any of them close to you. As A.P.S. members, they are bound by the society's rules and ethics, as well.
Selling stamps to a retail stamp dealer is still the most expedient way of selling your stamps.
are dealers on the internet that buy stamps as well. If the wholesale
value of your collection is very high, in the thousands of dollars, they
may come to visit you. In most cases though, you would have to mail
your collection to them for an offer. Most of these dealers are honest,
but sending anything valuable through the U.S. Postal Service, can be
dangerous, even if you insure it. This process can also be very time