I have a suggestion about something. If you buy a legit pet, then you shouldn't have anything to worry about, especially if the previous owner can confirm that. Just have the previous owner put on their lookup "(Insert date here) : Adopted out my amazing (insert pet nickname) to my buddy (insert your name)".
I'LL MAKE A SENARIO, COS I HAVE NOTHING BETTER TO DO =)
Let's say person X sells his legit pet, named "sonicboomboom69" to the user "pinkflamingo1997", who's name is Georgina. Have person X write on his userlookup
14/5/13 - Adopted out Sonic to my amazing friend, George. I hope you enjoy your new pet
That way, since it's vague and you don't reveal the exact username/petname, they won't be able to link person X's account to the pet, UNLESS they searched the cache. Once the suspicious PCer sees this, they'll be like "yep seems legit"