I was in an online community, with multiple PCs walking about from each of the players who had plenty of downtime, so this inevitably came up. The one in charge made a new subset of the Profession skill, Profession (shopkeeper), or alternatively Profession (salesperson) (effectively count as the same, just different names). You can make daily profession checks to sell stuff you possess, whether from crafting mundane items, magic items, or found gear in a dungeon, etc...
The amount (in market price) you could sell per day was (check result -10)^2, so if you take 10, you'd simply be able to sell a value equal to your profession bonus. To sell very expensive items, you could devote multiple days of selling until you reach the amount.
As far as bonuses to the check, he added a synergy bonus of +2 if you have 5+ ranks in diplomacy. Don't groan, it makes sense. And besides, diplomacy doesn't actually grant a synergy bonus to anythig normally. If you really hate boosting diplomacy, it works as well with sense motive.
He also gave circumstance bonuses for having a stall or shop: "A traveling stall (worth 200 GP) gives a +5 bonus to this check whereas a Store (worth a minimum 1,000 GP) gives a +10 bonus to this check.
More expensive stores in better locations can give higher bonuses. a 5,000 GP store gives a +15 bonus, a 25,000 GP shop gives a +20 bonus, and a 125,000 GP shop gives a +25 bonus. Anyone crazy enough to own a 625,000 GP shop would get a +30 bonus."
[sblock]He later changed it so that you sold an amount in sp, not gp, effectivle making it take 10x as long, because some people didn't like the idea of a Pc making 1000 gp a week, yet never once complained that the dungeons (which occurred weekly or bi-weekly) gave MANY times that amount. And also despite the fact that they had training rules that allowed one to gain hundreds, and later on thousands of xp in a week just by hooking up with another PC to train with. No risk of death as normal for gaining xp; and no feat expenditure, no lost money/xp, need to buy a shop, and so on that the item creation and selling rules required of you. It was...one of the issues that caused me to eventually leave the group over.[/sblock]
EDIT: In case it isn't clear, the profession check was put in to represent actually working to sell off your stock for full market value, as an NPC shopkeep presumably does as well. You could still sell without using the profession check and taking time for 50% market price, and if you had someone coming to you to buy something (like say another pC in your party), you didn't need to spend time or make checks to sell it at full value. Of course, in the case of two PCs, they probably make some kind of arrangement, such as a barter for something the buying PC has or maybe the PC wizard gives the Fighter a discount. But that's for the players to decide.