Depending on your playstyle or modlist, the mercantile skill can be either broken or useless. This mod is for users like myself who use Buying Game to make the skill worth its weight on your character sheet, but it'll probably work just fine for anyone else.

Regardless of your setup, you don't gain mercantile experience for making trades, only for haggling (and for bribes, if you have a mod that restores bribe experience). This didn't feel right to me. Surely, just like acrobatics and athletics, you should gain experience just for buying and selling in the first place? So that's what this mod does. You now gain a small amount of experience each time you barter, plus a very small fraction of the trade's total value. No more or less exploitable than repeatedly haggling the same item for good margins, just much more likely to influence standard play. These variables, as well as Buying Game compatibility, can be modified using the in-game MCM config.

This also means that whenever you're trading with someone who buys and sells at base value, such as creature merchants like Creeper or Mudcrab, you'll still get experience. Same goes for characters with high disposition (friends, faction members, etc) under Buying Game rules.

This is my first mod for Morrowind, but it seems to work just fine. I learned what I could from the code and documentation of MWSE and mods like Magicka Based Skill Progression and the aforementioned Buying Game, so thanks to them for that.