Mostly artist actually. If the artist doesn't have a good reputation chances are I won't buy.

I look at the artstyle. I prefer cartoon art because it fits well with Toribash and it doesn't get boring too quickly, unlike heads that are "too realistic" where there's really not much you can do with the look unless you got a lot of TC and can afford a high quality set that goes well with the head, plus the shading of the head would make it hard to buy a good matching force since it could either make it 1 or 2 shades too bright or too dark. That's why I tend to stick with cartoon art, the artstyle fits perfectly within Toribash and it's easier to obtain the look you want since all you really need is a colorful force and a head that matches it perfectly (And maybe some extras) and you're good to go.

I mainly look for heads by reputable artists (as long as the quality matches) or if it just catches my eye, uniqueness is a bonus.