Appraising stuff that I know how to appraise... I'll stick with suggestions for improvement, price suggestions if I'm really sure about 'em. :P
Zanzaba: Head is the best part of that set for me. The legs seem a bit cluttered and not too well thought-out.
mooman: I'd put it between 5-10k.
Serithi: It's a good start, but I feel like the theme could be developed. Fire demon? See if you can make the eyes more angry, and do a bit more with the legs.
athrax: I can't really judge abstract sets too well, but the heads on those two don't really work with the bodies, in my opinion.
sprytryne: Maybe they made transparency work with non-shaders in 3.8. I'm not sure. As for the new heads, they're rather pixelated. Try to fix that.
SixUgly: Without, but it's way too hectic either way. Simplify, simplify, simplify.
Disregarding mulguns, since he's banned for plagiarism.
takeji: The back of that head doesn't really work with the front mask area. If you could fix that, I'd put it at around 5k.
ZCW11728: I can't appraise tribal sets. Bit too hectic/theme-less, though.
nighthawk1: See above. Background pattern is nice. Hands are much too cluttered.
KungFuJC: Very nice. Transparency is a bit unnecessary in my opinion, but those could easily go for upwards of 5-10k, I'd say. The two human faces seem a bit too wide.
XxLanexX: Decent for your first, but that comment is overused. Proportions seem decent (though the eyes are placed rather oddly). You'll want to work a bit more before you start selling though, try developing a style. Anime heads are quite overdone.
Yourface: For one, you'll need to provide a 128x128 preview if you're planning to sell that. It's simplistic, I'd put it at 1-3k max.
sharkbate: First set: 10k at max. Head doesn't seem to work too well with the rest of the body, shins aren't mirrored, other parts don't look very thought-out. The overall theme is nice, if somewhat overused.
Second set: The head works a lot better with the set on this one. Personally I feel it's too cluttered, as I'm needing to work quite hard to see all the little details. What I do is pick a theme, and work off of that. Avoid adding unnecessary details that detract from the theme in general. 10-20k.