Oof thats a tough one? I've never really taken the time to think about the fact that it might be a buyer who wasn't like, actively targeting your specific pets. My first instinct was always ITS MINE IF I CANT HAVE IT YOU CANT HAVE IT, but I think it would really depend on who it got sent to? If it got sent to someone I hate, I mean like, sorry bud, you're going down (my grudges are lifetime commitments).
I might mail them and be like
"Why howdy there partner, it seems one of my pets has somehow made it onto your account and I cannot in the samhain figure out how! Could you possibly shed some light on the sityation?"
Or I'd just sit at my chair fuming and think of all of the ways I'd get back at them and do nothing.
Hope it all works out for you though :C