Do you care about swaying this particular artist or could you find someone who better suit your "voting with your wallet" necessities?
In case this is the artist you chose, then the best you can do is planting a seed with some feedback when you buy something. Pick the best one, or the one where you think the direction of the artist was better explored.
Preferably originals.
If everything is fanart (or you're not sure), then pick the one you're least familiar with and pretend you don't know it, praise her for her creativity and say something like "the fanarts are great, I love Sailor Moon, but I think I'm gonna get this one because it's original, and it looks just as cool."
If it's an original, it will give the artist a boost in confidence. If it's also fanart, it will give the artist guilt and maybe push her into a less derivative path.
Actually, I was just assuming it was a woman because of how retarded this art-selling LARP feels to me, but now that I look at the coomer undertones, I guess it's a guy.
If that's the case, then the strategy is different, pick the least interesting one, or least sexual one, and say you like it the most. He will second guess all his further attempts of drawing pure booba and hopefully introspect about doing something that sells for it's intrinsic aesthetic value.
