There was a time when stamps could be sent through the mail to pay for mail order items. I don’t know much about this except that it was a relatively common practice at one time. I suspect stamps were used to make small payments in lieu of change, which was too heavy to affordably send thru the mail or a check or money order written for an unreasonably small amount. Occasionally even today you will see on order forms a statement similar to "do not remit stamps," "stamps not accepted for payment," or just "do not send stamps." (The latter could refer to sending stamps to cover return postage, though.)

I know the practice is generally not accepted now, though.

My local post office WILL NOT accept stamps in return. I believe this is a postal reg or a policy, and would be very surprised if you were able to do so at your local post office. But you could trade the stamps to a friend for cash or something you need. Some businesses might buy stamps from you as a favor to you, although businesses are increasingly using postal meters, electronic postage or prepaid bulk postage permits. Try the business office at your church, mosque or synagogue if you have one, for example. A local shipping store or grocery store customer service counter might accept stamps if they are in resaleable condition. If you go to a school, the bookstore or other shop might accept the stamps, again as a favor to you.