Okay, by page 13 I think I'm going to skip to posting a reply. It's too early, and I'm sick.
Anyway, the whole notion of magic item shops and what do adventurers do with items they don't want to use anymore (*puppyeyes**whimper*) always seemed... daft to me.
What you do is you take your hard-earned bludgeld, and you buy yourself a decent little house for your party/crew. You have the wizard lay down as many non-detection , protection, and non-entrance spells/rituals as the guy knows. You have the rogue trap all the entrances and set up "someone's been here" cues (a la the old hair spit-glues to the door trick and whatnot). with your as-protected-as-you-can-currently-make-it home you then start stashing all the magic gear you don't want to use anymore. When you have a respectable collection, you hire on a hireling. You treat him nice, you
equip him with some minor magic equipment. You go on an adventure, maybe two. You pay him well (not so well you get boned out of your comfy Inn rooms, good meals, ale and whores, of course, but well). Then you hit a town, preferably the one you hired him in. And you tell him something along the lines of "We'll be resting up here for a a couple days, live it up".
Hireling #1 does what any young buck who's just been treated like a prize squire does.. he runs to his friends and relatives, showing off his magic gear and telling tales of the "Bold, daring, and generous adventurers" who have employed him, and flashes some gold.
All the while you continue to stash your unwanted magic items in your home, as unnoticed as you can manage.
When hireling #2 comes along (and this, logically, should not occur too long after hireling #1's first night of glorious show-offery at home), you equip him too, and pay him well. You bust your arse to keep these guys alive. These are NOT expendable NPCs.
And this, my friends, is how you build an army. A LOYAL army. Which is the first step toward winning a kingdom by your own hand. And honestly, if you can't think of a major role for every character to play in a "We rule a kingdom", then you're not trying hard enough. Worried about "Who gets to be The King?" Go the Narnia route and have all the warrior-types be the kings (a royal counsel, not unheard of, in stories if not in history), the religious types run the church(es) (which hold as much power as the king), the shady rogue types (depending on what kind of rogue they are) run the Thieves Guild, or whatever kind of thing they specialize in. The arcane types, obvious, become the much-feared Kingdom Wizard.
As for "Now we can't go on adventures, we have a kingdom to rule", that's what a seneschal is for. Arthur and his crew managed to find time to go out to war, go out on quests, go out on diplomatic missions, and all kinds of crazy crap. Your advisers (if you have any) may not like the risks to your life and limb you expose yourself to, but who's in friggin' charge here? Too bad for them!
Anyway, just my own thoughts on what to do in lieu of a "magic item shop economy", which I've always hated.