So, I was leveling this Ny'vene for Sairento but I was too busy to get very far with leveling so I set it back up for them again.
The pet was not in my party when it was set up for purchase, but I later added back it to my party before it was bought in the hopes of getting a bit more XP. I was not expecting it to allow me to add the pet to my party, as it was set up, but it worked.
I entered a battle with the pet and when I finished the battle, I noticed that the pet had been purchased by Sairento. I went to my party to add my normal attack pet to my party, because I was expecting the Ny'vene to be gone. Instead, it was still there. I attempted a battle without changing my party and it worked.

As you can see here, the pet is in my party.

However, this shows the same pet in Sairento's stables.


I have since removed the pet from my party and am unable to add it back, so I guess this is not a problem anymore but I can probably replicate it if I did the same thing again so I thought it was worth noting.

It looks like there was not a check for disallowing players to add a pet to their party that was listed for sale. This has been added so this should no longer be possible.

I will run a script to make sure that all current parties have pets that are properly owned.

Thank you for letting me know.