Used stamps exchanges.

Letters are ever harder to get, and the postal clerks don't help at all :(

Well, I think this wouldn't happen if you post your letters in person at a Post Office and ask for your stamps to be cancelled by hand. In most places this should be possible (if you ask nicely).

yes, would be nice to have such friendly post office. I did ask for hand cancel at my local branch (long before covid-19), the clerk looked to me awkwardly, but in the end he managed to cancel the letter. All the effort was useless: later I learned that they threw the letter in the automated sorter anyway.

but the automated sorter won't deface stamps with a black felt pen ..... it might over cancel them, but even that can be avoided by lacing the stamps in the middle of the envelope where the machine doesn't find them.

who knows, but it over canceled them. the red ink cancel that the clerk applied smeared the stamps.

Gluing postage stamps so that the machine does not stamp them is not a good idea. The official is obliged to cross out the postage marks found in this way.

Es ist keine gute Idee, Stempel in die Mitte eines Briefes zu kleben, damit sie nicht von der Maschine gestempelt werden. Wenn ein Beamter einen Brief mit ungestempelten Briefmarken findet, ist er verpflichtet, ihn durch Streichen ungültig zu machen.

Lepit známky doprostřed dopisu tak aby je nerazítkoval stroj není dobrý nápad. Pokud zjistí kterýkoliv úředník dopis s neorazítkovanými známkami je povinnen je znehodnotit přeškrtáním.

@ johnpeter what I was suggesting was placing the stamps in the middle of the envelope and then getting them postmarked at a post office.