Let's talk about the merchant for a minute.
Let's make him a pretty high level merchant, with magical items to boost his skill check and all. Say, he has Profession (magical item fence) at +20.
He should be making a profit of about 15gp per week at this, on average (half of his profession check). Assuming that he spends 50% of his income as operating costs (buying Identify spells, rent, an employee or two), that means his job brings him about 30gp a week.
Now, when I sell my +1 long sword to that merchant for 657.5 gp and he sells it to another dude for 1315gp, it would mean that it takes him about 22 weeks to find a buyer for a pretty standard, low powered item. That is much too long.
I'd say that the 50% thing is somewhat low. A higher percentage would be more reasonable. Unless of course the item is actually damaged, like CRGreathouse mentioned, in which case the value of the item would be diminished (and thus the price the merchant is willing to pay).
And there is no reason selling magical items for, say, 80% of market value would be unbalancing. After all, right now, coins and gems are inherently better than magical items as treasure, from a PC standpoint. And, what's more, magical items can also be used against the party, adding insult to (monetary) injury.
I remain unconvinced.
dcollins said:
Two-way street in both directions. Sometimes an NPC needs an item right now and can't wait. Sometimes a PC needs an item right now and can't wait. Sometimes not.
I had a feeling you didn't really mean to say that you screwed your players at every turn.
Staffan said:
On the other hand, a significant portion of the magic loot comes from NPCs, who generally have gear worth a lot more than the normal treasure for their level (e.g. an EL 5 encounter having an average value of 1,600 gp, but a 5th level NPC having 4,300 gp worth of gear). Even if you only get half value for that gear (and it's likely that the PCs can use at least some of it), that's still 2,150 gp - you're up 550 gp.
True, but that's balanced by the fact that a lot of monsters don't have treasure at all.
Trust me, in the adventure path modules (and you can't get more "typical situation" than that), this is a problem (unless you just happen to like weird, almost useless magical items). Of course, in a campaign with a lot of adventurer types as enemies, this would be pretty benefical for the PCs.
Both of these illustrate that it's important that the DM understand how "treasure per encounter", "encounters per level", "wealth per level" and "selling loot for half price" interact. A DM that understands these things can compensate for weird situations (such as the one presented by the default guidelines

). However, when a DM doesn't understand that dynamic, he can give out too much treasure (with lots of adventurer encounters) or find himself with a PC party with half equipment value without realizing it.