Back in the day, I played vanilla Morrowind without the Bloodmoon and Tribunal adds-on, and as I recall making money was kind of slow and tedious. Most merchants have a limited amount of money on hand to trade with you, and when you do acquire expensive things costing several thousand septims, you can't simply trade them for money because of this.

Enter the Dark Brotherhood through the Tribunal add-on. Once you find their underground lair, you pretty much got it made. Using a well-placed mark spell, recall, almsivi and divine interventions, you can pick off Dark Brotherhood guild members and sell their armor and weapons for a very fair profit. The armor saleswoman in the market district has 10,000 gold to do business with. Then I found out the merchant's 10,000 regenerates often, as does the Dark Brotherhood members themselves. So nice! A nearly uninterrupted cash flow.

If I remember right the helmets and gauntlets are 200, the cuirass 1000, the pauldrons and boots 500. Their weapons are worth very little at lower levels, but as you progress levels you find they'll start carrying glass daggers and even daedric tantos. Beware, because often times their weapons contain powerful poisoning enchantments, but if you're spending all this money on training and improved gear, it won't be a big problem.

Happy hunting.