MIDNIGHT ADVENTURER'S SUPPLY
Similar to thievery, but without the legal consequences, is the practice of lifting stuff from stores in the middle of the night. This system takes advantage of a "hole" in the game (it's not a bug because Bethesda knows about it and hasn't fixed it). The key to the system is the game's inability to "close" a store for the night.
The easiest way to make use of this system is to loiter in the store until after it closes. The merchant will still be there, but the shelves now function like your wagon does. You "remove" items rather than "select" them. Take the stuff off the shelves and put it on your wagon or in your inventory. If you're feeling particularly diabolical, you can sell it back to the hapless merchant (who is still functional even though the store is closed).
For a more profitable jaunt, note the closing times of the various stores, loiter in the one that closes the earliest, pillage it and sell it back to the merchant, then move on to another store, loiter, pillage and sell. Repeat until you run out of open stores.
To make even more money at it, you can run around town and break into all of the stores. The outer door will be locked, but this is no problem. There are three ways of getting into the store. You can pick the lock (which will bring you to the attention of the Thieves Guild, if you're not already a member), you can bash the door (which brings the guards running), or you can cast "Open" on it (which does neither - it simply opens the door for you).
Interestingly enough, the "Open" spell is close to 100% effective on exterior doors, no matter what your level or chance of success. To get the spell, you need to get access to a spell-maker. Set your chance of success to 1% + 1% per 2 levels (that's the cheapest version of the spell in terms of both money and spell points). Now go from store to store and rob them blind. When they open in the morning, sell everything back to them.
The problem with this method is that you have to leave town in order to get the stores to restock. The exception to this rule is the General Store. Although you may remove everything from the shelves, the next time you click on the shelf you will find a horse and cart there. There is apparently a major herd of horses in the store as well as a very productive staff of wagon-building fairies. The way around the restocking problem is to buy a ship. After cleaning out the stores, change your transportation mode to "ship". You will be transported to the deck of your ship. If you change transportation mode again (click on "ship") you'll be transported back to town in the same spot you were standing and the stores will have restocked. Please note that the last store you visited will have empty shelves (even if you didn't take a thing) until you click on the shelves in another store. I don't know what the purpose of this particular feature is aside from keeping you from pillaging one-store towns. Anchoring a "Recall" spell somwhere in town and teleporting OUT of a building will also cause the town to reset and stores to restock. Once you have a ship or a "Recall" and your "Open" spell, you can be a millionaire in a matter of a few hours.
Rusty Ogre Lodge: In the Daggerfall province is a town called "Rusty Ogre Lodge" the contains one Tavern. In the big upstairs room are several footlockers that will restock with Daedric weapons if you save the game and reload while in the room. Unless your Stealth skill is fairly high, opening any of those footlockers will summon the guards, but they cannot make the hairpin turn on the stairs, so will stay downstairs and leave you to your business. While you can try to fight your way out of there, a Recall spell anchored anywhere outside of the Tavern itself will make for a clean get-away. So for a quick buck (or high-quality weapons early in the game), repeatedly pillage those footlockers until your wagon is full or until you get bored (whichever comes first) and then teleport away to sell your ill-gotten gains.
Whichever method you use, exercise caution in your depredations. Cleaning out a town several times and storing everything on your ship until you can get to a place to sell it can cause problems. The game does not like to track a lot of items in your inventory. The moral of the story? Don't get greedy.