There are probably better ways to provide players with gold sinks than things like repairs, auction fees, etc. Spending money ought to be enjoyable, not requisite.

I actually disagree with this. I think it's neat to have some sort of "upkeep" cost in an MMO. You spend a bit of money on taxes and repairing armor, and buying some general necessities that you can only get from NPCs, etc. It's like your character is actually living in the game world and is helping support the economy that you, as a player, don't usually interact with.

Then there's player to player trading which can be enjoyable. Where you can buy stuff like crafted materials, rare drops, in Guild Wars 2's case I buy tonics and gems with my gold for toys/cosmetics.

I like the idea of an upkeep price where you're maintaining your character at the bare minimum level that acts as a gold sink, then the rest of the game being mostly player to player trading (where even then you can incorporate taxes in stuff like the action house). Makes the game feel more alive.