someone suggested that i should sell my vectors and i actually think it's a pretty good idea; maybe not the vector file itself, but if i sell and ship out prints/posters of different sizes. i have no problem doing this with my illustrations, since they're completely original, but i was wondering what the laws are regarding things that are already copyrighted.

for example, if someone wanted to buy my ironman vector in poster form; is it illegal to sell something like that if it's based on something that really isn't mine to sell?

That sounds like it would be illegal, but let's see what someone else says.

It is illegal. Copyright infringement.

I think that if they were vectors that you did freehand that it would be okay. I mean, DA sells stuff, and I have seen some prints there that are based off other images from movies. Since other people are saying it's illegal, stay on the safe side and don't sell ones that use copywritten images.