Well, that's the interesting thing. You likely can import and use these wheelchairs, but they can't be sold as medical devices and, subsequently, won't be covered by your insurance.

The dumb thing is these super cheap wheelchairs would likely cost the end users more money than the expensive ones as they will buy them out of pocket vs having their insurance/medicare cover most/all of the cost.

The reason these companies don't sell these wheelchairs in the US is likely because the marketing would run afowl of regulations about selling medical devices. You probably could sell these as bicycles and hobby chairs though.