I would buy it if it were in my area. That's a big machine for the price. Something like that in my area would go for 6000 (USD though) easily. Ask them for a recording of the vibration, we might be able to tell what's wrong that way. Causes could be anything from a bent ballscrew, bad coupler, bad motor alignment, bad support bearings. Worst case you would have to replace the entire ballscrew assembly which is cheap from china, but shipping will probably be killer. Given it's length I would just retrofit the machine with a rack and pinion.
I would go to see the machine, watch it jog around and see what's up. Maybe have them disconnect the y axis ballnut and move the gantry around by hand just to make sure the problem is with the drive mechanism and not the linear rails.