I would return the amp, get a full refund and insist the seller pay the shipping back to him.
If you're going to keep the amp, take it to a tech, get a full estimate/diagnostic and ask the seller to pay that ( actual repair cost ). Just getting that estimate is probably gonna cost you half an hour labor or more.
Even if that amp only needs $80 in parts, if it takes the guy 4 hours to fix it at $110/hr ( the avg rate here for a tube amp tech ) then you're gonna be highly pissed.
I had a long drawn out clusterfuck with a guy who tried the arbitrary 'It will cost this much to fix' bullshit and he insisted that that's what it would cost to fix it even though he was basing that on a different amp with a different problem. He was too fuckin cheap/stupid to just take it to a tech and get the facts. IF the PCB is cracked or the OT is hosed. It'll probably cost more than $260. Find out before you agree to his offer of $260.