Does anyone know wich merchan buys them? I got an inv full with them. Or is it only 1 time?

Officer Vai in Bereghost (the woman who also wants bandit scalps from you) buys them for a very good price.
If you already brought enough scalps to her so she is gone you can sell them to the mage at high hedge.

thanks im glad im not encumered for nothing ;p

You may not be able to sell them if you're playing the modded game. I have BGT installed and Officer Vai will not buy them, and you can only get rid of one head at the temple (quest). I didn't try High Hedge (never thought of it).

Again, keeping in mind my game's modded, there is a fellow in Baldur's Gate (west or southwest, IIRC) that will buy one, and I believe you may be able to sell some at the Sorcerous Sundries in BG.