A non-functioning accelerator pump (an internal part of the carburetor) in an old simple car or truck won't prevent it from running. All it does is squirt extra gas as you press the throttle, like a kids squirt gun.
With no accelerator pump at all, it would stumble and run lean on acceleration. (like a lawnmower does). But using the choke would substitute the same function, so he should be able to start and demo the tractor for you.
If he insists its the accelerator pump, you insist he start the tractor using the choke. Watching him do it should give you a good idea of his mechanic competence.
He's not selling a tractor, he's selling a yard ornament that an experienced mechanic couldn't fix.
I would line up a parts tractor first. If that takes a year to find - well, you just learned how rare the parts are. No hurry -he won't sell this one anytime soon.