So I am in Velen now and im doing all the Questionmarks and
these give quiet a bit of loot...
My problem now is that the merchants in Oxford and other
small villages never have enough coins because I have them.

What can I do to avoid that ?
I tryed meditating but that doesnt really help..

Any merchant business cannot survive by buying alone. He must sell to make a profit. It is no different in this game. If you unload all of your gear, but don't furnish him with coin in return, of course he will run out. There is a 5 day (I believe) turn around for his purse to refill, so you will just have to go elsewhere, or spend as well as sell.

Mh I see...
Waiting 5 days gets him just a few coins tho, guess
I will have to spend some coins <.<

Thank you