Check your Quest Log and see if you have the "Butcher of Homlett" title. If so, I am not sure there is anything legit within the game you can do to get rid of it. You might just have to suck it up and do all your business in Nulb or with the Lareth and the bandits in the Tower behind the ToEE (if you let Lareth live that is).

As the game progresses, you should be manufacturing your own magic item equipment anyways, so no big deal not to trade in Homlett.

Much later when your MU gets the Dominate spell, I think you can dominate any NPC and still trade with them (it is actually a massive cheat, because you can sell something to the NPC, then give it back to the seller and sell it again ad-infinum). But I can't remember if their store inventories update properly once dominated.