As @rajillustration says, there's a general understanding from indie sellers who have small online shops or table at conventions not to sell fanart of series who are also small indie creators. Most indie creators are also pretty receptive to a creator going "please don't sell fanart of my stuff" although that itself can cause some problems when fandoms start policing themselves (looking at you SU and Homestuck fandoms). Even larger series can generally come to an understanding. It's considered bad taste to sell Moomin merch by its fans, for example.
Generally, as long as they're not making huge amounts off of my work, I wouldn't mind. As some have said, it's free advertising. And as long as they're a small little artist who just draws fanart and goes "hey maybe people would want this".
Your much bigger problem comes from, and if you've ever been to a larger convention you've probably seen them, the larger sellers who take fanart without permission and print it on wall scrolls and sell them as if official, or as if they have permission. I've seen Yuumei's art of Sabre (the most common but many others) sold at conventions all over the place and you have to report it every time but of course no one does anything.