The way i see it with this issue is that you cant really think too logically with it. Yes, i agree if nobody sees you take an item ,then in reality, you should be able to just skip town and hawk it for coin. But the problem is that the game becomes unbalanced and i doesnt even pose a challenge to break into someones house or even a castle private quarters and just grab everything.. I believe the way BGS has designed it ( in oblivion) was a decent way of balancing it.
The problem isn't really that you can sell your stolen goods wherever you want. The problem is that it was too easy to steal things. Instead of making the thievery system engaging and difficult, they decided to add an artificial limitation on how you could sell stolem items. The way to balance the stealing system should be to make it difficult to steal things, not difficult to sell them.
My proposal:
First of all, lockpicking should be in real time, and you should need a high security skill to reasonably open hard locks (if there's a minigame, don't stop time, but do make it more dependent on your security skill. Maybe there could be a perk that slows down time while lockpicking). Sneaking should be more dependent on lighting and sound as well as your sneak skill.
Commoner houses should be easy to break in, in areas with low guard activity, and contain very little valuable loot (maybe only 1 or 2 slightly valuable heirlooms). The bourgeois should have houses in better guarded areas, with harder locks, and perhaps some loud guard dogs, but you can find more medium loot. The aristocracy should have houses in extremely well guarded areas with difficult patrol paths, very hard locks, guard dogs and hired guards, and the valuable loot you find should be something you really have to work for. Places like castles/palaces where the highest of the nobility live should have very good, non leveled loot from the start of the game, but it should be extremely hard to get. The should be guards all over the place, and the areas where they store the best loot should be off limits to the general public, requiring some serious sneaking and lockpicking to get to without being caught.
Merchants should get very suspicious if you go into secluded areas in the back of their shop, especially if you're sneaking around, and they should ask you what you're doing or kick you out if you're being very suspicious. The majority of shops should have at least one hired loss prevention guard (not just a repurposed city guard but more like a mercenary).
Common items below some threshold value should be unsellable only to the person you stole it from. Perhaps very valuable or unique items could become unsellable to honest merchants in the city after some amount of time depending on their value (maybe a week for 1000 Ivalue, 24 hrs for 5000, or something like that). But I don't really think there should be news of the theft spreading to other cities except for maybe some extremely rare, unique items, and I do think that every city should have some less honest merchants that will buy something even if they know it is a stolen item from the shop next door.
In fact I really think the two things that would balance the stealth system better than the thieves guild fencing system would be guard dogs that can bark and alert nearby guards, and lockpicking that doesn't stop time. Honestly the save system is partially to blame, as my thief characters almost never get caught because I can just reload my save if they do. This lets them easily get just about anything they want. I think the best solution would be to make it almost impossible to steal valuable things with low stealth skills, so even if you reload a save you probably won't be able to snag what you want.