They sell expansions, as well as aesthetic items
Basically this. The $30 price is to keep spammers and gold farmers out, but you get quite a bit of game just from that default payment. Only two more content packages have been released since going B2P (they call them Issues, like City of Heroes) and you got the first one free if you signed up before January 2013. You also get a free month of premium when you buy the game, which gives you access to said content for free for your first 30 days. Going premium gets you rewards (a unique costume piece each month) and free points to spend in the Item Store, among other things (like the aforementioned content access).
The also make money from the normal F2P stuff like pets, costume items, XP/AP Boosters, and such. Most of the stuff you can buy in the item store is unique to the store, though a few of the older costume pieces are recolors. For instance, the blue poncho you can buy is the same as the green one you get for free if you do all the missions required in Blue Mountain.
They also get a cut of the merchandise from the
WeLoveFine store, which sells exclusive TSW clothes and such. You also get free stuff depending on how much you buy. Buying one shirt/item gets you an in-game title, buying two gets you an in-game shirt.
So really they are doing pretty well with their sales and the game is usually on sale if you look on Steam. Buying from Steam gets you a cat pet that boosts your stats in the early game and a ring that gives you more XP during the early game, as well as a Steam t-shirt. Buying the physical box from Amazon gets you a suite of useful (and balanced) starting weapons, faction related leather jackets, and a raven pet.