This is a good ethical line to hold, honestly. It's also why you see folks like Critical Role or Homestuck have to create fan art policies, because comic cons will fill up with unlicensed art and it actually can be enough to put them out of business, whereas you can have 60 artists selling Wolverine prints and Marvel won't feel it. It does suck when you're selling original work and the booth next to you is making bank selling unlicensed Batman prints, but that's as much on the attendees only wanting something familiar and unwilling to try out something new as it is on the artist selling the prints. My first show I was talking with one artist, I'm blanking on his name right now but modestly big name, was complaining - everyone in artist alley complains, it's our national sport - and he looks back at his signage that is ALL unlicensed IP work and he just shrugs and says "I'll bitch and bitch but I know where my bread is buttered" and points at his Emma Frost.
So much of artist alley is just playing chicken with C&Ds, though. I'm still trying to figure out how Zenescope and Jamie Tyndall got away with White Widow for as long as they have (and I like Jamie, I tabled next to him two shows in a row, not a knock on him, just wonder what magic his legal team works). I mean holy shit his selling Magik as Mutant Sorceress Yellow and Emma as Mutant Diamond Icy
on his site right now.