Are used stamps worth anything after usps postmarks?

For any collectible to be worth anything, it must be rare, unique and/or wanted by somebody else. Nobody is going to pay a lot of money for something they can buy anywhere, from anybody.

Stamps, if they are kept unused and in mint condition, can maintain their face value and actually increase in value as time goes by.

A used stamp, if it is still in good condition and the postmark is not too intrusive, is still worth something to somebody.

Many of the most valuable stamps in the world have been used, but they were made many years ago and are considered rare and/or unique, that is where the value, if any, arises.