Different editions of D&D have set selling prices to 20% to 50% of the list price (not counting haggling and bargaining systems).
OK. I'll get back to this below.
We have found that in order to not focus on hauling loot a lower percentage works best, for us. (At 50% I have at least one player who can't resist spending the whole session on figuring out ways to bring back every. single. scrap. of inferior armor, various knick-knacks and even low-grade ore. Making looting a non-issue seems to help him focus on actually having fun...
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In our crew we're nearly all like that - a greedy bunch of remorseless murderhobos except without the hobo part.
Yes, random. In my example, I made rolls on the DMG magic item table. In actual play, I'd do pretty much the same, but with one eye to serving the actual characters and their classes.
List prices: you can use the DMG. I use Sane Prices.
Which DMG? There's been a few...
Not once per whole adventure, but once per visit. Between visits you need to perform at least one quest or mini-scenario. This would pretty much boil down to once every session for us, since we level up every three sessions or so.
So, no account for in-game time passage if the party's just hanging out in town for the winter? I'd throw in a time factor - something like either once per adventure or once per month, whichever comes first.
Treasure division: my players have never had any issues with this. They have an internal round-robin list where you get to pick an item (and be placed last on the list) or keep your spot (but get no item).
Once everybody passes, any remaining loot is sold for cash. Cash is then split equally.
I DMed a party that did it this way once and it turned into a nightmare. One character always drafted the highest-value item on the list, no matter what it was, then would sell or barter it into what she really needed...and pocket the difference. She ended up stinking rich compared to the others in the party, leading to an imbalance it took years to correct.
Also, a headache with items having lower sell prices than buy prices is that now every item has, in effect, two values - a low and a high. This can make equalizing treasury division very messy - is a claimed item given high value, or low value, or something in between? Every time an item is claimed, or a claim is dropped, everyone's share changes. It got so bad that I-as-DM eventually put my foot down and dictated (very artificially and arbitrarily) that the sell price and buy price of items would henceforth be the same (so, a single-value system), and that's still how I do it today.
Your system would maybe make this even worse, as an item might now potentially have many values. Ouch!
Lanefan