I got one keg from someone on CL for a song. It wasn't holding pressure but the guy had no way to fill it with gas so I took a chance. Replaced the o-rings, cleaned well and removed poppits only to find gunk. I cleaned well, replaced everything and it is holding pressure right now. the other from CL had new o-rings, guy was still in the HB business, and was holding pressure when I got it. I simply rinsed it well, checked out the poppits and it seems okay.
Kegs from Kegconnection were rinsed well but that is all. Kegs from AIH had to be completely cleaned as they came with soda inside of them but supposedly had new o-rings. They did recommend poppits so I bought a set of those just in case.
Basically the name of the game is to clean everything well, replace o-rings unless a company says they did, and then fill with gas, spray star san to see if there are leaks, leave it with pressure but not on the gas until I am ready to sanitize again and use it.
I always hook up to gas first after I fill it with beer, release gas, let it refill and spray with star san to check for leaks again. Put it in the keezer, hooked to gas, and the next morning I check to see if there are leaks again.
The key is leaks, obviously. So far I'm doing okay with this method.