So I'm going to check out a Peavey 6505+ tomorrow and the owner said the tubes have been replaced and sounds as good as new. My worry is that the person replaced them with cheap tubes and might have to replace them somewhere down the line. Does anyone have any idea how much that cost? It's at a good price and i might be a little paranoid about it. What are your thoughts?
You couldn't possibly replace the tubes with anything cheaper than what Peavey puts in them from the factory. I wouldn't worry about it. You could always just ask him what kind of tubes went in.

A full retube for a 6505+ would run you about $150-200 with standard modern tubes.
Ask what tubes are in it.
Ibanez SIR27
Pod HD500x

Mesa Boogie Roadster 2x12 combo
Cmatmods analog chorus, phaser, tremoglo, signa drive, butah, and deeelay
walrus Audio Descent