I installed the CEP, and now trade mode never innitiates anymore. After the appraise check, the camera moves, but nothing else happens - the merchants' inventories do not appear!

That's odd.

Try the corrupt file fix from NWN Vault.

http://nwvault.ign.com/?dir=cep/downloads

What module is this on? It's probably best to contact the module's author about it, since he'd be responsible for putting CEP in.

The corrupt file fix is for fixing specific haks which the program would say are missing. But my program doesn't say any files are missing - it simply doesn't open the trade mode.

The modules i tried it in were the "Dark Ranger's Treasure" {abandoned it when i realized it's just a pointless shorty anyway} and "To heir is Human". is it possible that in other modules the problem won't repeat?

If the modules aren't set to use CEP, then others whould be fine. That's assuming CEP is to blame, of course; I don't see a reason it would affect stores anyway. If the OC stores don't work, then it's not CEP's fault, and a reinstall would be best.