I personally disagree with this idea, since people will start begging for karma, like people said above. And when people create items and markings it wouldn't be fair since the person will have to pay for it and then give others the same item pack or markings to see them. For presets and meshes it would be fair since the buyer won't have to give the other player only the .fhp file and for meshes nothing, maps too.
And there is a somehow karma paying, since most of people karma the creators of awesome meshes, items, markings, presets and maps.
So I see there is this thing already, thought Karma being as a currency wouldn't be good, it's better being here as a ''Thank You''.