Aria Silverhands said:

Players will try to abuse the system no matter what you do. Hence why DM's are seen as power hungry control freaks. They have to be to prevent the idiots from ruining the game.
No, that's the point of "adventuring" in an mmo. In D&D, the point of being an adventurer is adventure. The stories they discover, the stories they're a part of, the stories thrust upon them unwillingly.
So what? Railroading is great. It's awesome. As long as you're not a damned idiot and tell the players you're railroading them. Railroading is what keeps the campaign from being ruined by stupid decisions by stupid players.
Because they wouldn't play. That's why great dm's railroad their players without the players ever realizing it.
Maybe I'm just lucky, as I've never had any of those problems. That's not an issue with the game, that's an issue with your players.
Most players want to be challenged. They complain and get bored when monsters are too easy to kill. They want to use their cool new powers and they want to feel that they are useful.
If every combat they go into goes like this:
Player: I roll a 6 on my d20 to hit. That hits? Wow these monsters have really poor ACs. I do 35 damage.
DM: Alright, it dies.
Player: Really? In one hit? That was easy. Let's go find some real monsters.
then players will get bored. The goal is to challenge them at the right level. Not too hard to get them frustrated, not too easy that it is a cakewalk. If you give them too much money, eventually monsters will reach the cakewalk levels.
Sure, most players will tell you that they LOVE the feeling of being way too powerful for their level and knowing how quickly they can defeat monsters. Don't believe them. These same players will be the ones complaining in a couple sessions that the game is too boring. Plus, it is unlikely to be fun for you as a DM to continually come up with interesting encounters only to have them defeated without any real chance to shine. I know I hate rolling dice when I know the monsters as so far outmatched that they'll never win.
Being a DM isn't about being on a power trip. It is about having the responsibility to ensure the fun of everyone at the table. This means making sure no one feels that their character is too weak. It means making sure everyone has an equal chance to shine. It means making sure you are having fun running the encounters. It means coming up with an interesting storyline that entertains your players
And sometimes, to ensure everyone has fun for the long run, you have to tell some people no. Sometimes that means telling someone they can't buy an item, they can't have a certain feat, or they can't be a certain race. And other times it means keeping their gold value balanced any way you can. Whether that means limiting the gold in the future or coming up with reasons why they can't get more than 1/5th the gold when they sell an item, it is part of your responsibility. If you do it well, they'll never even notice and they'll just have a lot of fun.
Really, I've found the most effective way to provide a challenge is to think WAY outside the box. I've mentioned it a few times, but I once threw a freaking HUGE giant at the party at one point for storyline reasons. And it was killed almost entirely by
one player, not because he had buffs and was overpowered out the wazoo, but because I didn't expect the paladin to put on the crappy belt of climbing and re-enact Shadow of the Colossus.
Side note - that paladin
earned his Holy Avenger rip off.
I think the problem is, the players ARE going to notice they're getting ripped off the second they try to buy an item after selling one. As for controlling their items, we rarely use the Magical Walmart approach; if a player can give me a good and believable story as to how they're going to purchase the item they want, we'll roll with it. If they just say "I want a new sword of flaming awesomeness," no go. On the other hand, if they (and this is an example that happened) talk it over with the rogue, and the two of them tell me that the rogue got in touch with some old contacts and hooked the first player up with a disenfranchised wizard hoping to make a quick buck, I'll start going with it. And the best thing is, they're all but tripping over themselves to give me more ideas for plot hooks

. Later on, that same wizard found himself much more powerful, and was feeling a bit irritable that he let his awesome sword go, and goes after the party. Or maybe I nudge the fighter while they rest at the inn and say "Hey, roll Listen." And of course, later, "You notice your sword is missing." Of course, I leave enough clues to allow them to look after it, while not so many that there's a big neon sign saying "ROGUE STOLE IT, MET WIZARD HERE."
I suppose the idea of taking control away from the players just doesn't seem right with me. The game isn't supposed to be about the DM playing the game by himself, nor is it about the players just dully responding to yes or no questions, followed by combat. Again, there's video games for that, and they do it better. You've got a group of people sitting around you - let them really get involved in it!