Looking at buying a used Peavey Triple XXX

Dude on Craigslist is selling the head with a Marshall JCM cab with a 1960's badge on it for $600:guh:

What's the catch? I'm going to take a look this week. Amp noob- what should I keep my eye out for? The mid knob on the lead channel is busted off and the pedal acts up on occasion apparently... Shits old and a little beat up but it is what it is- that just seems like something too good to pass up.
 
Make sure the cab has good speakers in it, get more familiar with working on amps and you can easily repair that knob and if the problem is in the actual footswitch, that should be easy too. Make sure the amp comes with tubes!