From Diablo Wiki
A "gold sink" is a game term for a feature that gives players something upon which to spend their gold, usually in the end game. Gambling was one gold sink in Diablo 2, but it was not useful/expensive enough to retain long term use, and many players eventually came to regard gold as basically useless. The D3 Team is aiming to include many more useful gold sinks in Diablo 3, as a means to keep the economy functional.
Nothing is yet final since the economy remains a work in progress. Jay Wilson did confirm  that respecs and item modifications would cost a lot of gold. In addition to those, it's assumed that Auction Houses will have a gold cost associated with them, and most players expect that gambling will return, in some enhanced (and expensive) form. As item repairs and mounts are not part of Diablo 3, two common forms of RPG gold sink will not be found in the game.
- Gambling in Diablo II. Money sink.
- If you want to get into, buying anything from any vendor is drawing wealth out of the economy and therefore could be considered a gold sink.
- I know what you mean though, you’re talking about the huge $$ investments that don’t provide immediate benefits to your character. Which, by the way, ground/flying mounts in World of Warcraft wouldn’t figure in to as your increase in income from simply traveling faster more than makes up for it. But that’s not really important here…
- We’re attempting to build a more stable economy in Diablo III, and just like Diablo II did, we will have ways to draw money out of players. If that means there will be big ticket items or not, I don’t know, could be. If we want to throw out gold as much as Diablo II did we’ll definitely need some better places to spend it if we want gold to be worth anything.
- To address the original post, we don’t like the idea of binding items just because Diablo is a trading game. To take away that ease of trade but add an asterisk behind it stating “Unless you pay a lot of money”, it’ll just make it feel... I don’t know… kind of crappy. There are better ways to go about keeping a stable economy, and whenever possible they should be on things the player finds helpful, useful, or at the very least “fun”.