It would take players who care more about buying/selling than exploring/slaying.
Magic-Merchant PCs are likely to spend a lot of time sitting around guarding their inventory, looking to offload it while defending against thieves and other threats.
One of my players recently said "We only sell at 20% of list price? Screw adventuring. I'm going to get rich by selling magic items."
My understanding of the D&D magic item economy as it stands is that it is more profitable for PCs to be out killing things and taking their stuff than it is for them to try to craft or sell magic items, so the opportunity cost is high enough that PCs soak the pawn shop economy prices as part of doing business.
One other relevant factor is the level of fun inherent in these things. I know that I went into my current Traveller game with a thief / merchant with the intent of buying and selling many things for a great profit... and while a great profit I did make, it was actually a lot of work, and kind of ceased to be fun (so now I've moved to only dealing in high-value cargoes like radioactives).
In my Ptolus campaign one of the groups of adventurers found a treasure trove of life-sized, hand crafted, very detailed trees made of precious metals. They resealed the dungeon entrance with magic until they could buy a shop in the city. They then carted the trees back to the shop by magically shrinking them. Their target audience of course were the wealthy merchants and nobles. Took them more than a month of in-game time to make it happen, but they got the job done.
I've always considered the magic item market to be one of high overheads. You've got to have really secure premises, you've got to have spell casters with identify and remove curse on stand-by, and most of your stock sits idle on the off-chance that one of the few people in the area rich enough to afford it wants any of it.
So that shopkeeper is only offering you 20% because he's got a couple out the back anyway, and it's going to cost him another 20% securing and identifying the thing anyway.
All that's missing to make the economics work properly is a "used gear" market model. That way, the merchants would not be selling used gear at new prices...and PCs could get used gear for less than full cost.
Of course, as used gear, it shouldn't be in as good condition, so there should be mechanical disadvantages- not as good an AC for armors, breakage rules, something.
I've never felt that D&D had a functioning economy, period. It just really starts to break down, especially when you consider that for the last two editions, the only thing that gold is really good for is buying magical items.
Am I the only one who thinks 50% makes sense? I mean IRL shops buy the items they sell at 50% (or less) too.
While that shouldn't put you off into doing something for your own game, understand that economics doesn't work well with D&D, in fact is counter to the way the game is played. So it's really something not worth considering.
I always saw the guideline as just that. If you are willing to wait a season, hire a seller, bring my items to a large trademeet, getting people interested. A +3 Bane (Evil Outsider) sword may have its hilt reworked by the Priests of Whacabalor for their Guardian of the Eastern Reach. They may even pay 80% of the original cost... But you are currently 1000 miles away. Do you want to pay to Teleport over to them, do all the Diplomacy and Knowledge (Religion) checks, and then travel with your loot?
Didn't think so. But that merchant will. That merchant makes his money on it. He probably is part of a trade union that has contacts all over the world. You're paying for their agency, and they say so. Now would a merchant possibly give an offer for well-traded items? Yeah, if you work with him and offer him first-pick. He may even offer you some work on the side... But since most people treat the Magic Shop as a 7/11 with portable holes they don't do this.
Simple System (add the following percentage of your overall sale to your overall cost, Except for Pawn). :
...snip...
1 month, 20 days: 24820 (26620, - 1800 GP overhead).
Can your party wait a whole 50 days for that cash? If they can they'll make 7k more than what they would have. If they wish to accompany to the location? Heck, there's the overhead cost in pocket and the chance for more adventures.
For purposes of this hypothetical campaign, let's say the answer is "Yes."
Thank you for providing a solid answer to the question asked in the original post!

(Dungeons & Dragons)
Rulebook featuring "high magic" options, including a host of new spells.