Well just look over at the "The By You/For You Policy" since they are really hard when it comes to things like that.

Far as I gather, as long as your not breaking copyrights(like by directly copying something and selling it as your own) you should be ok. I mean, remeber walking through Otakon and the dealers with characters in key chains, sketches, and even a very beautiful Ninetails and Vulpix kit painting.

I would personally follow these guidelines: http://help.deviantart.com/743/

Since it is fanart, and sadly not original content, just a different take on an already copyrighted character, you cannot sell it for profit without prior consent from the original copyright holder.

Don't want you getting in trouble for something you may not have known about.

However, if it IS a character based on a series, such as an anthro Vulpix from Pokemon or whatnot, as long as it's mostly original, that can be sold.

What about something like Harley Quinn where she's a character technically owned by a company but even in their realm of sold works drawn by multiple artists to the point that she has no specific style or artist behind her?

I would say if it's a specific enough character to have a name attached that is a personality, then I wouldn't do it.
For instance, Harley Quinn is a very specific person.
Pikachu is a breed of Pokemon that there are now almost 700 different types of.

...but Pikachu is a very well-known and iconic character of the Pokemon Franchise; not to mention one of it's trademarks. In this instance; Pikachu would be the worst choice of the two XD

On the other hand; something far less known; like say a Hoppip; it's a Pokemon; but not really super-popular.

Now; as you were saying before; something derived from the original that contains components of uniqueness; that's different.

Yes, Pikachu is popular. But, Nintendo and Game Freak aren't as likely to sue if you create a print of your own character that is a Pikachu anthro, as compared to what would happen if DC got wind of you selling pics of Harley Quinn without their permission, even if it's your own art and take on the character.

I depends....I think if you're doing just fanart and as long as you have stated it is not your character, you might be fine. Usually people are spooked on doing fan art and selling it, but with more people selling works of copyrighted/trademarked characters at cons, the threat of being sued by a company is pretty low. It's when you try to take credit on the character that becomes a problem. You're not doing something crazy either, like, trying to make a Poison Ivy sexdoll or anything like that, so I wouldn't be too scared about someone coming down on you for drawing and selling a picture~