I bought some second-hand kegs that came with replacement o-ring kits, but the o-rings for the posts gave me issues. They seemed to fit on fine, but then I started having issues with the QDs not wanting to come off. This is despite lubing with lubri-film. Either the o-rings were too large from the start, or they swelled up enough to cause the binding. I'm betting the seller just used some parts bin o-rings that "seemed to fit" and not ones specifically designed for keg posts.
I swapped out the "free" o-rings with replacements from a reliable homebrew supply store, and I haven't had the issue since.
On tap:Witbier, American Stout, Imperial Stout, Root Beer
Bottled:"Massacreation" Chinook Barleywine, Belgian Golden Strong Ale, English IPA, Cabernet Sauvignon
Next: US IPA