I like your rules suggestions here in their general structure, though I wonder at what counts as "loot" for the following:
"Selling Loot
When recovered items are in perfect condition, they may be sold at half of market price. If they are in poor condition, or filthy, they might only be worth one-tenth of regular market value."
It also feels like there's something missing - intrinsically valuable loot (gold and emerald necklace for example) should probably be possible to move for a lot more than 50% of it's value, even to a merchant, if it's at least obvious it came from adventuring.* To a fence, definitely 50% is reasonable of course, if that (20% even). But perhaps that's not what is meant by loot?
I also note that by using the maximum possible DC (25) quite readily, and 15 for things that are merely "rare" (surely in most cases these will not be found asking around, but by going to a shop or market?), and putting all the weight on Investigation, it makes the route to and type of characters who can obtain this pretty narrow (Wizards and Bards, basically, and the former will be better at finding stuff than the latter in most cases, which seems rather implausible).
* = This does open up a hilarious thought of "adventure-laundering", where you mix the results of actual thefts and so on into the results of adventuring. I hope one particular player in my group never thinks of this.