I'd liken the trade in magic items more to a black market, and an illiquid black market at that. I think equating them with drugs or guns in our modern market might not be too far from the truth.
First up, our traveling merchant has to give up the initial 20% outlay. Then, he has to transport them from town to town, so the magic item has to pay a percentage of the cost of the wagon, horses, feed, hands and wear and tear. In travelling, he needs security, probably increased security now that he's carrying a magical item. Then there are bribes, taxes, tolls, local charges for setting up shop in the market, etc. The Merchant's Guild takes a piece of his business as well. Also probably a good idea to pay the local Thieve's Guild their vig so they just don't steal the thing from you.
THEN, the magical item is a highly valuable and risky object to move. So, the Merchant probably deals with the negotiations personally. Now, you can't just put up a sign '+1 sword 1000gp, no lay-by' for this sort of item. One, you'd just asking to get killed and robbed (maybe even by the local authorities in less LG areas). So, you're going to want to make discreet inquiries among reputable buyers who might not slit your throat rather than paying for the goods. Hiring guards to look after you while you're doing all this is also going to be expensive since, again, you want reputable people who are unlikely to slit your throat and take the goods. And you want to pay them well so they don't feel like they're being ripped off.
And we haven't gotten to haggling with the potential customer yet, spending all that time before being told 'no, it's too expensive' and then having to go through it all over again.
Now, if your PCs would rather turn all that into an adventure in itself, more power to you. But it's probably a more efficient use of a party's time/wealth ratio to raid tombs of loot and eat the 80% markdown. After all, the only way anyone would consider selling a magical item is if they were of no use to them anymore, they're just too good to give away on a whim.
Now, if you assume that two groups of Adventurers are going to meet, it presents an interesting dilemma. Imagine an encounter between two groups who are just as avaricious and grasping as each other, even if both are the 'good guys'.
If this group is of lower level than your PCs, they probably can't afford what you're selling and the PCs probably don't want anything they have in trade. If at equal level, then a decent ammount of barter can probably be had, trading useless items for more useful ones (although, if you think they're useless, chances are they're useless to the other party too). If the other group is of higher level, the PCs probably can't afford what they're trying to flog off and they run the risk of the other party beating them up and taking their stuff.
(am I discussing economics or ecology? I can't really remember anymore...
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And then we come to religious orders, organizations and nobles. Now, if your Paladin wishes to bring his religious order into DnD economics then I propose that we re-institute the practice of Tithing to the class. I remember, playing a Paladin for 20 years or so, the joys of having to give most of my loot to the orders, along with any magic item I couldn't wear. I'd LOVE to inflict that on 4E just to displace a certain amount of anger and resentment. The same goes for the Order of Mages, the Thieves' guild, Assassin's Guild, etc. And, I think Adventurers should be taxed along with all the rest of the Kingdom's citizens.
As a DM, I'm starting to get a headache just considering the prospect of having to run a deep and realistic DnD economy. I can imagine the reaction of my players at the table if I rocked up and said 'Hey, guys, I don't have an adventure for today... SO instead, your characters will be doing their tax returns! Now, since you've adventured in The Free City of Greyhawk, Waterdeep and Sharn thanks to numerous Plane Shifts, the interdimentional tax collection agency from Sigil is threatening to expunge your essence from the multiverse unless you get these done tonight! So, in other words, if we don't do this, we start a new campaign..."
Yup, that'll go over REAL well...
