Good luck.... Its a REALLY touchy subject on this and many other threads. Probably the only way you can be SURE its a legal keg would be to buy it from Sabco for an arm and a leg. Honestly, if you yourself don't do the deposit w/ no return, you can probably limit your guilt on the purchase. You can't fix what has already occurred. The way I did it was call around to some local breweries who dist. their own barrels and just ask if I could buy a couple of their kegs that didn't hold pressure any more. I drove to the brewery, had a few pints with the manager, he gave me a personal tour of the whole brewery...and I mean everywhere, and then gave me two kegs at the end of the whole thing and said to donate the 35 bucks a piece to charity.
Thats my suggestion. Most smaller breweries are more than happy to dump their scrap to a homebrewer.