There are a couple good ways to make money in Oblivion.

When selling, always try to maximize the disposition. If you have any charm spells, use them on the merchants. These are not considered attacks. You can also buff your charisma. Then, play the mini-game if the disposition isn't already at 100. Then make sure that you haggle. The best way to do that is by selling arrows 1 by 1 until you reach the point the merchant refuses, then back down by 1.

Firstly, alchemy is a good and steady source of money. The Imperial Market area has a lot of containers that respawn ingredients. Guild Hall tables respawn. Pick up ingredients, make them into potions and sell them.

In the Mage guild halls, there are alchemy tools laying around. Once you're an initiate, you can pick most of these up without stealing. Pick them up and sell them right back to the alchemist.

Order of the Virtuous Blood quest... once you have a fame of 5 or so, you'll be approached in the Temple District. You'll have to sit around for a while. You'll be sent hunting a vampire (just listen to the guy in the hunting cabin, don't kill him). Once you complete the quest, you'll become a member of the Order. Best news is that they reward you with 100 gold for every Vampire Dust that you turn in. There are a lot of vampires out there. WHENEVER YOU TANGLE WITH VAMPIRES, MAKE SURE THAT YOU PRAY AT A SHRINE. Otherwise you'll turn into a vampire yourself.