What kind of handlebar should go on a fixie to sell it? I've seen fixies with mtb handlebars, sawed-off mtb handlebars, drop bars, and bullhorns.  The fixie I built has drop bars, but I'm wondering if I should change that if I want to sell it.
Handlebar type and shape is largely a matter of personal preference - especially on trendy fixed-gear and single speed bikes.

I would sell it as is. If you change the handlebar there is a likely possibility that you will also have to change the brake handle. That's a lot of needless expense. Let the buyer swap it out if they want something different.