Winter Veil is probably one of the most profitable game holidays. There are lots of achievements and quests people do, and some of these involve acquiring things that can be sold on the Auction House. The most interesting thing is that none of the things people will be needing require a maximum-level character to get and are, in fact, farmable by just about anyone over level 6 or so.
Let's start with Small Eggs. Five of these are used to make the five Gingerbread Cookies required for the holiday quest Treats for Greatfather Winter. Small eggs are easily farmed, but the demand for them between Dec. 15 and Jan. 2 is more than can reasonably be farmed by any one person. People in the know have been preparing since last year and have stockpiled these eggs.
That said, the bulk needed to meet the demand generated by just about every single active player doing this quest on all their characters is pretty high, and these eggs don't come up on the Auction House that often for cheap. For the next few days until demand drops off a bit, people will buy these for between 2g and 5g, and if you have a character capable of killing level 6 monsters, you can farm hundreds an hour in Eversong Woods outside Silvermoon.