This?
At this point, given your time frame, I think that JB Weld and disclosure to the buyer is your best bet. There are aluminum "Welding rods" that can work like solder to do repairs like that, but given the proximity to the canvas, paint, and plastic it would probably be better to just remove it all and have it professionally welded or replaced.
Is there another flaw in the top mechanism that placed stress on that? If so, a reattached the ball will be short-lived.