It doesn't matter how you assume "other adventurers" exist, the fact of the matter is you never want your player party coming into contact with something resembling another player party.
And why the frakety-frak-frak not???
This throws off the whole aspect that the players are the main focal point of the story. Sure there are people out there doing similar things, but even without predestoned futures the player's PCs are more important than any NPC because it is a story about the players. Anything that deviates and gives a higher focus to some group other than the players are involved in has been a bad judgment by the DM for a roleplaying game, and would be best to just write a novel about his NPCs.
The PCs have to realize, however, that they are not the only adventurers out there; if 4e is trying to imply that they are, that's a design flaw that shatters the game.
The PCs, like it or not, operate in a world - whatever world that may be - that has in it people less powerful, as powerful, and more powerful than they are. When they go to town to get a resurrection cast, for example, where did the casting Cleric get all her levels from? Adventuring, perhaps?
Which means, by default, that there's going to be other adventurers out there. Sometimes, those adventurers might compete with the PCs for the same resources...two groups, for example, trying to be first to reach some goal; or two parties destined to fight each other for some reason (warning, though; those combats are a bugger to run!).
And, again by default, if there's other adventurers out there it only stands to reason there'll be some direct contact between them, allowing the possibility of direct trade in magic items maybe even at full price.
PCs sell to merchants. Even if they place it in the guild for commission they are effectively selling to a merchant or a fence, and the book says 20%.
Who said anything about placing it in a guild? I'm a Fighter, in town for a break, and I've got a spare Flail +2. I stop by the local mercenaries' guild and bump into Hjorth, another Fighter from up north, who has Flail as his <insert favoured weapon mechanic name here>. Horth just picked up a nice suit of scale mail +1 and I need new armour; mine got melted last trip out. An obvious trade situation, without any of this 20% garbage.
Technically true. The player party are the only "adventurers" and everything is is just NPCs, unless you start swapping items form one RPGA venue to the next. I have yet to face an NPC adventuer from the MM myself.
Huh?
What's the RPGA got to do with anything?
If the PC party bumps into some Dwarf, asks the Dwarf what he does for a living, and gets "adventurer" back as an answer, then bang goes that theory regardless whether or not said Dwarf ever joins the party, then or later.
4e wants PCs to be special flowers...but let's not take it too far.
I think the PCs should be the only things called adventurers even though there could be a in-game guild called this. Another hard part of discussions where in game and out of game one word has different meanings. I think it also extends beyond just 4E and even D&D where the "adventurers" as it were are the players groups of characters.
If it's levelled, has a class, and makes its living by killing things and taking their stuff, it's an adventurer. Whether it has a player attached to it or not is immaterial.
And, to drag this back toward the topic, adventurers are likely to form like-minded guilds (thieves, assassins, and wizards in particular) while clerics have it built in as part of their temple structure. Once this happens, there'll be trade and talk within those guilds...stories will get told, information will get shared, and items will be bartered.
Lane-"haven't paid my guild dues"-fan