I'm not saying you shouldn't expect good seals you should. What I'm saying is that vendors aren't taking the kegs apart and visually inspecting each and every poppet, oring, and relief valve. They are just cleaning them and applying pressure, if it holds pressure , then they sell them. Once you buy them, and then clean them and replace the orings, taking them apart, you may dislodge residue or realize something is just too worn or brittle.
If after cleaning/applying keg lube, they dont hold pressure , I would contact the vendor and ask for a replacement or parts. But I would not expect them to know the condition of the parts because I know they were not torn apart prior to sale.
I'm not trying to stir trouble , just trying to inform everyone of the process and condition of most used kegs. I've bought kegs from 4 different vendors, and I've had issues with all of them, but not issues that were not taken care of or solved with orings, keg lube, or a return.