I've had a few people buy my pets, and then when I navigate to their page, it says "this pet has been released". Just wondering if it's some kind of unspoken pet sharing etiquette that I haven't heard of before now?

They have all been genetically tested, so I know it's not that. The pets I put up for sale aren't that great either, but I like to put them up in case someone wants them to breed for traits or colors, when I would otherwise just release them myself. It's like they are cleaning house for me, but are paying to do so, LOL.

During festivals there are event where people earn points if they relase a certain kind of pet, if they do relace a lot of a speacie they may get a themmed from that same speacie.

Oh, gotcha! Totally not something I would have thought of.

that help cleen some of the pets with no trait that are for sale, otherwise the number of those will increace forever and almost nobody would buy them. If your wory about one of your for sale pet, just make sure to increase the price or remove it during the big festivals ^-^

Outside of festivals, there are some players that do buy and release pets. Generally to clean up the pet market, so usually they do this to pets that are very cheap and/or have little or no traits.