[sarcasm]Magic items are really rare and valuable. Just like all other items that are really rare and valuable, there is absolutely no interest whatsoever in anyone buying and selling them.[/sarcasm]
I didn't vote because there was no option that appealed to me. There are no magic shops in my campaign, but there are stores that have among their wares a magic item or two. Because of the party level and size of the city, those items at this point are limited to 8k and under.
There is one shop -- run by a tiefling -- that supplies the PCs with gear, both mundane and magical, but I keep track of his stock so the party can't just do the equivalent of going to Wal-Mart (a running gag in our modern games). But this is not a "magic shop" per se. This is a shop that sells a limited amount of magic items, among other things. Because the PCs now know he's a tiefling, they don't completely trust him, so they don't want to keep handing items over to him.
Why do I allow magic items to be bought and sold? Because magic items are prevalent, and they're valuable. And someone invariably is going to get an item they can't use or already have. Or they outgrow them when they get a better item. What then? Throw that valuable item away because no one will buy it? Heck no, sell it or trade it in for something else. Why would a shop buy a used sword +1? Because they can sell it to another person.
Since I control what is in stock at these shops, I control what items the PCs can buy. If I don't want them to get a ring of invisibility, then there won't be one available. But I don't want to roleplay every single time they go shopping. That's not why I play.
Friday's session in my campaign sees the heroes' return to town after partially clearing out a hobgoblin stronghold. The first thing they have to do is have about 2 dozen items identified, then split the loot and pay off the mercenaries. They want to gear up and return to the stronghold, so I've had them spend the week coming up with a list of things they'd like to purchase or have made. I definitely don't want to spend an entire gaming session on a shopping trip.
I also have no problem with PCs selling items for half price but having to pay full price for them. This helps me better control their wealth levels. And it totally makes sense. That shop owner has to pay rent to stay in business, and that means turning a profit. When you sell a car or a CD, you get full price for it, right? Wrong. You get some credit, and half price is being extremely generous. I'm now trying to steer my players toward commissioning items, so they can have items more closely suited to their wants and needs.
So no, there's no Magic Wal-Mart in my campaign. But I'll allow the PCs to find an item or two at shops around town. If that wasn't possible, they'd all be loaded down with +1 daggers and longswords that they can't use.