I don't sell my pens at shows or fairs, so I can only offer my own opinion.
If you are willing to find decent paper to write on, I'll take a fountain pen 9 times out of 10.
Rollerballs are next up for me as long as I'm not writing on SUPER cheap paper. They'll do fine on standard printer paper and most notebooks, for example, particularly if you use a fine tip.
I avoid using ballpoints when I can. If I have to use one, the Schmidt 9000 is far and away the best one, though of course it only works for pens that take a Parker style refill. I think I even prefer the Schmidt 9000 to a gel refill, though I also like a Monteverde gel type.
I'm making a pen for a left-handed coworker, and after trying all the fancy fountains and rollerballs and whatnot, he said a ballpoint works best for him. It's what he's used to, he didn't "catch the bug" of using a higher-end writing implement, and instant drying = no smearing for him. So I'll be making him something that takes the Schmidt 9000.