You're only drawing a background. As long as the scene isn't also distinctly part of a copyrighted or trademark universe created by the same owner of the rights to the character, you're not liable for anything the other person adds to it. If the person is trying to do a parody or satire piece with both the character and your background, then neither of you has anything to worry about.
Show me one example anywhere in American case law where someone has been prosecuted for "aiding and abetting copyright infringement".
I don't mean actually facilitating it, like they're saying of Kim Dotcom who is charged with actually providing the means for illicit file sharing, but by doing something as incidental as this. You almost might as well prosecute the art supply shop for selling Copics to the infringer.
Hence the sentence: None of this will likely ever get litigated, but it's worth noting.
Also, one should never underestimate how many charges can be leveled against just about everyone involved - especially when it comes to the sue-happy nature of folks such as the RIAA/MPAA and other rights holders.
They tend to have a shotgun approach to litigation. And in the case of Kim, sometimes "shotgun approach" is taken literally.
This isn't true everywhere. Parody is protected under Fair Use rules in the US, but that's not always the case in Commonwealth nations or the UK. Last I heard, parody was very clearly NOT covered under Fair Dealing in the UK. You might want to double-check.
The art that you're being paid for will not contain anything copyrighted, correct? So you can't be held responsible for anything that's done with it by the person making the purchase, as I understand it. Sort of like how if someone used a bolt cutter to steal something, they wouldn't go after the hardware store?
It is actually illegal to use fanart for anything other than personal use, like putting up a drawing on your bedroom wall or turning in fanart for a school assignment. The only reason nothing is typically done about it is because the law requires the copyright owner(s) to take steps against it. Either direct action against the infringing party (i.e. a DMCA notice) or giving expressed public non-consent (like how the NFL has their disclaimer right before broadcasts), but most tv/comic/anime/manga studios see fanart as free advertising, so they don't do anything about it. In most cases. Selling official merchandis will get them to wake up and take action against a person, but for fanart made by individuals they'll only do something if they feel it's necessary, but that is not the only thing that copyrights are against. Venues that allow the selling of art typically don't allow the sale of fanart simply because it's a potential liability for these issues. Not because studios go around like hawks looking to sue people, but because some prefer tighter control than others and the venues can't be sure that ever piece of fanart being sold isn't just a copy or tracing of original studio art.
No you said it was only illegal to sell fanart. I said that's not the only illegal use of fanart, and then explained why. If a company said their characters were not allowed to be made into fanart and sent dA a letter notifying them, dA would have to take down every single one, regardless of whether they were being sold or not. Strictly speaking in legality, personal home use (or school assignments, depending) is the only legal form of creating fanart.