Those Yamahas were supposed to be half decent there's a few posts I was checking out the other day in here, from a year or two back at least. Also the Koss should have a lifetime warranty if they are listed at the KOSS SITE, and have cheapo pads I think $5 a set (?) for the models also at thier site. Sure why not probably even cheaper than a thrift store if you could even find all these together. Be fun seeing what's up with the AKG if it just needs soldering.
I say you got a good deal, and it will also let you check out their differing sound qualities. Finding pads I'd say go with cheapos, maybe find sizes and check at Koss for most of what will be comparable. At least you know they are decent and cheap.
Good score. $40 is nothin all said and done.
Did he throw in plans for a neck brace attachment for your chair with those things in the picture? If they are anything like the Sansui SS-20 I have they are ridiculously heavy after a bit.
Oh I see according to your profile you are a " Self proclaimed professional amateur " so you should be A ok with these. 
Edited by nick n - 9/30/11 at 9:36pm