I'd buy it. Kegs don't go cheap these days. Even if it was acquired through questionable means it's already cut and useless to the brewery who rightfully owns it. Unless the seller is stealing kegs and making them I'd say go for it.
Is it cut? Are there holes drilled? Any hardware on it?
Around here, 15.5 gallon sanke kegs sell on Craigslist for around $50, uncut. For $70, I'd expect it to at least be cut.
$70 to save that trouble would be worth it, IMO. The fact that it is not drilled is a plus. Better to choose what and where you want to mount for hardware.