Are you sure they have 250-350 miles or 250-350 hours? The "miles" would mean they are brand new with less than 20 hours. That would surely make them on the high side of value near $6000 or more. If it is hours, I'd say they would sell for $4500 to $5500 depending on condition. Our 2004 Mule 3010 is quite nice and I would sure not sell it less than $6000. Of course, it has some additional options like a windshield, roof, and bedlift.
On any of them you look at, make sure they go into and out of 4wd smoothly and also into and out of rear differential lock. 4wd should engage immediately, but differential lock requires you roll 10-20 feet for it to engage.