Yep, it's normal.
One of the earliest cheeses ever posted after BG2 was released on various posts was the note that you could wrack up a fortune stealing and reselling goods to the Thieves' Guild, while stealing and/or buying the best stuff outside of a few merchants that didn't allow it. It's not a cheat, since it's in the game mechanics. It is a matter of unbalance, and taking advantage of it is far too easy. So is the result. I get no pleasure out of circumventing a game's rules and a real zing out of winning by whatever conditions the developers set--but that's just me. You buy it, you can do with it as you like.
