If there is even one player against thieving(as much as I am for it) it should not be implemented at all, against PCs. NPCs like Harnquist could be robbed, however. You can only loot what has been sold to these merchants. This would create a dynamic where the thief would follow someone who has been killing trepors to the merchant to loot the merchant afterward.
There are skills in place for this already. They could be utilized to not get caught because if you are caught: jailtime! This could be an instant port to a jail cell where you have to do in game jail time.
Quests could be set up so that NPCs sometimes receive items that are valuable for trade to other items(glyphs, weapons, etc.) that can only be gotten through the quest(like the energy glyph now). This sets up hot spots for the thief to \"case\". This also creates a dynamic that multiple thieves casing the same place draw attention to the group. The thieves then have to roleplay how to deal with that.
Another dynamic could be thieves can rob from other thieves. The option to thieve can be turned on, and anything stolen is marked as stolen goods(to prevent turning the option off to protect your stolen goods. At this point, you could sell stolen goods to a PC and that PC is subject to it being ripped off(no other item, so shame on you for receiving stolen goods). Guards could stop you and confiscate your stolen goods, creating a need for fences that can remove the \"stolen goods\" mark.
I can go into a lot more detail, but I think this sums it up for a system of thievery that satisfies the thief in me, without violating those that do not want to be targets for this practice.
Kythag