Personally, I wouldn't go anywhere near a 5yr old console. It's more than likely to be a console which has already RROD'd, and then fixed. It's likely to have a fair amount of use too. People buy them up cheap as knackered consoles, repair them and sell them on for $$$ profit. Because they've previously RROD, the dashboards haven't normally been updated. They then RROD again a few months down the line. The fact he knows how to replace the thermal paste and X-Clamps, suggests to me he's doing exactly that.
Also make sure he replaces the GPU heatsink with that from the late Zephyr's, Falcons. Also make sure you get a photo of the idle tempertures in FSD2 before making payment. WTF was selling a Xenon on here, and have the fans 12V'olted, Heatskinks replaced, fan shroud mod. That's the minimum for a Xenon console.
Much also depends on how much you are paying for it. WTF's Xenon for example was pretty cheap, and so was worth the risk. But if you paying too much, you're better off looking else were for a newer consoles. I'm sure there will be people selling them in the netherlands.
Replacing the X-Clamps has been known to help, as has using artic silver. But it doesn't permantly fix the issue.