Two professions...

For a one-boxer, probably Enchanting and Jewelcrafting.

Even without the supply of transmuted gems, there is a steady profit from buying and cutting gems and then listing them; I can often make a profit just from prospecting.

If I charge for an enchant, I make an ok amount. If I enchant for free (their mats) and accept tips... 25-30% don't tip at all, 5-10% tip a lot more than I'd charge and the rest tip 5-10g. I've made 500g an hour, at times... leveling my enchanting with other people's mats.