Its just what the topic says...
Its good they made an option aviable ( to disable map transparency )
but they forgot to make such option for inventory. You know its uncomfortable to check loot while i see unnecesary background.

Hrm - don't think it is possible to be honest, I suppose you could try hacking through the data files to see if you can find the semi-transparent texture used, then replace it.. but there is no straight forward way that I have heard of. Sacred wasn't very mod 'friendly'

Unfortunately, it's probably not possible at all.
There is no option for it in-game, and you also can't do anything about the data files, because (as of Sacred Underworld) everything is hard-coded into the .exe file.

I don't know how, and I don't know why you'd want to. If I'm trading with someone and somebody attacks me, I wanna' know about it.