I would be inclined to replace the potentiometer or to see if you can attach something to its shaft.
I don't know how good with your hands you are but the first thing you would need to do is to pull out the amp. You'd have to do it anyway if you decide to go the whole amp replacement route.
When you do it, find out what brand the potentiometer is, may be you can find the replacement on the web or Radio Shack. Also, when you take it out maybe there is enough of the shaft length left to attach something to it. Hard to say without seeing it, so just carefully remove the amp and go from there.
On the side note, it's hard to believe the knob got cleanly sheared off in the shipping. I would suspect a rough play from the previous owner. Can you contact him/her? I don't know what good would it do, though.
Good luck,
Gena
