That is a reputable auction site. A couple more are:
You can also buy directly from some transportation contractors like Laidlaw or First Students .
I have purchased from all of the above except Laidlaw and been very happy with my purchases. The only exception was a foul tempered lady at Govdeals gave me a seriously bad time for contacting the seller and arranging to pick up the bus after Thanksgiving. Seller was happy with the arrangement. So was I...... I got my bus.....
First Student took me on a test drive and took time to go over maintenance records. Really nice folks at their Seattle location.
Good luck with your quest.