There was that one artist who, 3-4 years back, produced a card set which he was selling at the San Diego Comic Con, of painted images he did of Marvel, DC, and Star Wars characters. I won't mention the name, but we discussed it here on the forum.

Each year he'd release about 10 new cards, and I think he got up to 30 or so. Seems like he stopped, as I haven't seen new ones in 2 years or so. I don't know how he got away with selling those.

I see tons of unlicensed prints available for sale by artists at conventions. I have heard there have been crackdowns here and there on them.

Same thing happens with commissions, there are lots of licensed characters used, but its not a big deal when it's one offs and its quiet.

It becomes a big deal when people start advertising and mass producing to the point that it comes to someone's attention. A little money can afford to be lost, but a license holder would be foolish to let anyone else cash in a lot on their copyright. Some are more blind than others, but you don't fool around with DC, Marvel or anything Disney.