The way I see it is, the original content of most fandoms is the stories, so selling a fan story is too close to ripping off the original creator. Obviously it's more complex than that unless your fandom is literature, and some of the original creators do make money selling copies of art, but for the most part, fanart doesn't infringe on anything like what the original creators produce.
Aside from the ethics portion of it, there's also that fanfiction can have questionable quality. Even if there's a good sample, it could fall apart in places or be bad around every other part you read. With fanart, it's really clear what you're getting because you can see it all when you buy it, you're not going to find out that the bottom half of the picture sucks because the editor only really bothered with the front half. This is a risk when you buy any books, but with professional ones you can be fairly certain of consistent quality and have an easy way to return it.
YMMV, those are just the reasons why I'll buy fanart, but wouldn't buy a fan novel (except when published professionally). I do think it's perfectly cool for fanfic writers to have a pay-what-you-will system or request donations, though.
(People also have odd ideas of paying for stories at all. EG libraries are common, but usually have really small movie sections, and why there are movie rental companies, but nothing analogous for books.)
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Aside from the ethics portion of it, there's also that fanfiction can have questionable quality. Even if there's a good sample, it could fall apart in places or be bad around every other part you read. With fanart, it's really clear what you're getting because you can see it all when you buy it, you're not going to find out that the bottom half of the picture sucks because the editor only really bothered with the front half. This is a risk when you buy any books, but with professional ones you can be fairly certain of consistent quality and have an easy way to return it.
YMMV, those are just the reasons why I'll buy fanart, but wouldn't buy a fan novel (except when published professionally). I do think it's perfectly cool for fanfic writers to have a pay-what-you-will system or request donations, though.
(People also have odd ideas of paying for stories at all. EG libraries are common, but usually have really small movie sections, and why there are movie rental companies, but nothing analogous for books.)