It\'s the size of the object stolen and the person from whom it was stolen. Theft could work as long as there are limits. Master thieves shouldn\'t be concerned with the new guy in the corner hoarding his 2 gp. Those guys are small potatoes. Thus a level limit on the low end would protect newbies from master thieves.
Ok, here\'s what you need if you include pickpocketing.
#1. A Fence. Someone who buys stolen goods and sells them at a profit. He pays the theives (or the Guild Master for being able to run his business). Fences don\'t care who they sell to and usually don\'t care what the item is. Although if the item is too hard to get rid of, the fence won\'t buy it. Leaving the thief with...an item he doesn\'t want? Sure that leaves room for griefers, but what if after a certain period of time, news leaked out that you had something that was stolen? With something small like, say, an amethyst, it\'s no big deal. Probably about to be used in a tradeskill. But the sword of Uberyleetness is gonna stand out. And the item should stay in the database for a while as being stolen. The fence can\'t take an item that\'s worth more than a certain amount, because he can\'t sell it, because there aren\'t many buyers for rare items other than players. And if the rare items have a flag for stolen on them, certain authorities should find out and start asking questions. (aka possible GM intervention?)
That brings me to my second point.
#2. A rumor monger. That\'s right, this guy loves to get the dirt on everybody. Including PCs. So Mr. Thief comes in with that sword of Uberyleetness and wants to sell it to the fence. The rumor monger sees the fence turn the thief down, and rats him out for a fat tip. And possibly the gratitude/protection of the person the thief stole from.
Which brings me to number three.
#3. A code of laws that penalizes certain actions. This could include jail, possibly pillory time (getting stuck in a wooden[stone?] contraption while everyone sees who you are and knows what you did), or maybe more drastic measures. Of course when you have people carrying out those laws, you come to:
#4. A couple of corrupt people who are carrying out the law, but are also on the take. They periodically let important people out of jail, for reasons they probably better keep secret.
Still, to avoid all that time and effort being gruesomely stripped from him by a potential competitor (nobody said the rumormonger couldn\'t also be a thief), he\'s prolly not going to grab the biggest, most expensive item he can find. Probably the smallest, most expensive item he can find. It\'s a lot easier to sell something small and turn a profit than to hoard something big and hope for a buyer.