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It's assumed right through 3e and 4e that PCs can buy and sell magic items by default.
For the entire length of 3e, we were talking about ways to avoid using a blatant market. There's been lots of success in that area.
The real assumption in 3e is that the PCs will have access to magic items, and that at a given level, they'll have in the neighborhood of a given amount of "wealth" of items. How you manage to get them those items is entirely up to the GM.
This is not objectively bad design, but I do not like it. However, if I just eliminate it at a stroke, PCs will very quickly have too much money (as defined by me).
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Again, I simply don't want to change out the amounts of treasure given. Firstly, if I start giving out tens of silver where before I was giving thousands of gold, I will very rapidly have a mutiny on my hands, arguments about 'relative amounts' notwithstanding.
Any mid-stream change is apt to be a bit awkward.
If you've been giving out XP the normal way all along, and suddenly change to, "Sorry, no XP until you spend your cash", you're apt to see much the same resistance. The players will see, in no uncertain terms, that they're spending $$ for XP, when they didn't have to do that before, and they will feel equally robbed - "Hey, we beat the monsters! We earned the XP! Now you're telling us that we have to buy it off you, rather than spend the gold on stuff we want?!? What's up with that?"
Secondly, discovering huge hoards of treasure is cool.
Yes, it is. But even Bilbo realized that he didn't actually have use for all that much of it, nor could he effectively cart it around safely.