i thumbed through my own faves to check, and yeah, i've faved about 2 or 3 characters that i had my eyes on for buying if i feel i can work them into something.
otherwise i will just make a mental note to myself that i like a design, but not enough to buy it. usually if i see a design i like AND i have story potential for it, i'll go ahead and just buy. i do have a private list for design links i'm into, but there's very few there.
imo as long as a person isn't very.. obnoxious, i think is the word, about their faves being a wishlist i don't see the harm. people usually put up tags or descriptions saying characters aren't for sale, and if someone adds a non-ufs character to a wishlist, its on the fave-giving person. i put characters ufs and people end up faving them for what i ASSUME are potential buying / wishlist purposes, and i don't really mind it.