The water pump does not interact with the oil sump - he did the correct gaskets - perhaps a cracked head or leak in the coolant galleyways. He should have had the head and cylinder magnafluxed as part of the gasket changing process.
Only pay what it would be worth needing a replacement engine. Which it probably does!
He's selling it having poured money into fixing it with no success so either he knows what's wrong and its bad or he doesn't and its hard to track down
You'd better know for SURE what the cause is before handing over any money. You don't want to end up replacing the engine.
crack somewhere inside the engine, somewhere (keyword: somewhere)
I'm looking at buying a 1992 yamaha FZR 600 and the owner says that he discovered coolant was getting into the oil so he stopped riding it. He replaced the cylnder head and base gaskets but that didn't fix it so he's thinking that it's the water pump gasket. What do you think?
