I think it's more the artists/people selling are assuming the risk is smaller with a small run since they're making less money, and therefore less likely to be targeted for copyright infringement. I'm sure the copyright holders could come after anyone, regardless of the run size
Popping in my IP attorney head to say, yes - the copyright holders can come after you at any time.
Before I get into this, I also have to provide the caveat that I am not your attorney and that anyone reading this post should not take it as legal advice. Please consult your own attorney before taking any action.
Traditionally, the rights holders for nerdy properties have been relatively lax with people making their own fan art and selling it. Mostly because it'd be really bad publicity for Disney (or whoever) to run through Artist's Alley at a Con dropping C&Ds on their most ardent fans. But that doesn't mean that a regime change at Company X couldn't change that.
Also, keep in mind that actual profits aren't the only means of collecting damages from infringers. Assuming the copyright has been registered (which it likely has for any pop culture property), the owners can opt to seek statutory damages of up to $30,000 for non-willful infringement or up to $150,000 for willful infringement. And since it's hard for anyone to legitimately argue that they didn't know STAR WARS was copyrighted, I'm sure they'd seek statutory damages if going after a small quantity infringer.
Finally, the prevailing party in a copyright suit is eligible for attorney's fees. So the theoretical cost of filing a suit isn't what's stopping the rights holders either.