Hey Veritas,
I have an idea that's been knocking around in my head for a while. It can be done (I think) with vanilla resources plus some scripting, it adds some realism and roleplaying value, and it is relevant to one's thieving experiences - so I thought I'd drop it here and see if you wanted it, since it would make a cool TIE feature if you liked it.

So: Why is that guards can ALWAYS spot stolen goods? It doesn't in general make a lot of sense - unless, I suppose, the stolen items have distinguishing marks. Perhaps that dagger I picked up has a name carved into the handle. Perhaps that ring had a house symbol engraved on it. That magical scroll might have the creator's signature, that book might have a bookplate.
In real life, such marks of ownership can often (not always) be erased, scratched out, modified beyond recognition. Serial numbers can be filed down, old bookplates can be steamed off and new ones glued on. So, it would be nice if the player had a way to do that too. In particular, it's always annoyed me that it's essentially impossible to steal adventuring equipment for yourself; sooner or later, the guards catch up to you, take your stolen gear, and turn you out of the prison without pants on (embarrassing). If it was made sufficiently expensive/difficult to do this, it would not be unbalanced; the player would then only attempt to claim ownership of items that they really cared about, rare items that they wanted to wear, use, and carry. I think I've come up with a reasonably balanced (balanceable?) way to implement this, too.
The fences of the game (and possibly the smiths as well) could sell "jeweler's hammers". These hammers can have a legitimate use, which justifies their sale by smiths: They can be used to reduce jewelery to gold/silver + jewels (easy to do; chance of success based on armorer + luck; failure means either that the hammer breaks or that the gems are ruined) or to turn raw materials into jewelery (considerably harder, again based on armorer + luck; failure breaks the hammer). Their
real use, however, is that they can be used to remove the "stolen" status from any equippable item, and perhaps (maybe with more stringent success conditions) from keys as well: one uses them to obliterate any identifying marks on the object in question. The player's ability to do this is without breaking the hammer is based on a roll against armorer + security + luck. These jeweler's hammers can look exactly like repair hammers, but scaled down.
For books, "fine parchment" may be sold. A possible legitimate use of fine parchment is to allow the player to enchant his own scrolls, or to keep a diary; I don't know whether either of these would be easy to implement, but either would justify sale at magic shops, if desired. However, if you have fine parchment, a quill and an inkwell, then you may be able to remove the stolen status from a book, by providing it with a new, generic bookplate. The chance of success could be based on intelligence + security + luck, and any attempt destroys the parchment itself. Alternatively, fences could sell pre-made "forged bookplates", which cost more but always successfully remove the stolen status. These can look like either the vanilla parchment or the vanilla scrolls.
Of course, in both cases, these may be classified as thieving tools, just like gas arrows and lockpicks, in which case they would be sold only by fences and confiscated if you had a run-in with the law; they do not need to have additional "legitimate" functionality. Between the legality status of these tools, their cost, and the chance of successfully using them, they could be balanced to almost any game difficulty or expected cost. Additional constraints could be imposed by causing a brief but powerful stealth debuff on use, or a prohibition on use while trespassing, to prevent the player from abusing these tools while in the act of thieving; also, of course, use on quest items would be prohibited. I also think that this is a feature that would be appropriate to include in an overhaul, since it does affect the global balance of the game where thieving is concerned.
I hope you like (and maybe implement?) my idea! If you like the idea but don't want to script it

, I think I can figure out enough to provide at least the very basic functionality as thieving tools that I've suggested here.