I wouldn't be surprised is Blizzard owns one of the sites which is selling the gold. They could even create characters with available transfers and sell them on without anyone even knowing.
I think it should be allowed because of exactly what Ghostsuit said, it gets a bit boring farming for raids every day. And it can turn the game into a daily job. Just farming the materials for a raid that night. It would be interesting to see if there is any members of one of the top guilds, Nihilum, Forte, Deaths and Taxes etc if there is any member who doesn't buy gold. And if he doesn't; how much time a day does he/she spend farming materials for the up-comming raid. You would probably get some surprising numbers.
There is ways to stamp it out though, and in my opinion it is to make a Guild Economy much more doable, before it was created by allowing guilds to take on random players at the cost of a few hundred gold to instances like Molten Core and Blackwing Lair. But that hasn't been fully established yet since TBC. And it might be a little harder to achieve with the smaller ammounts of teams which are required for each instance. If they could add something in the instance which will allow guilds get large ammounts of gold back for doing certain bosses (Especially the first kill, as it's usually the most costly). Then i'm sure guilds would be able to fund their own members for raids a lot better.
Oh and, they didn't make the Epic Mount 5000G for nothing !
