I haven't traveled very long distances for gear, max 2 hours. But I remember when my brother some five years back drove 1000 km in one day in an old crappy small Peugeot when he bought an old Fender Rhodes from a town on the other side of the country. Those are pretty hard to find so I guess that justified the long trip, especially in our country that's not really known for being densely populated...
If I would go for a similar trip for some bass gear, it would have to be to buy an expensive old vintage amp or bass or a new custom made bass.