I get that, but me as a newer player, I wanted..gold (to buy avi items, equipment, fancy looking/themed things, philters, etc.). So I bought pets and I sold them. The issue with that "set to level" suggestion is that a player can purchase the pet and sell it, and a more higher-level player and purchase that and release it. Or if the player released it because they're confused about which region gives a boost to Themed Pets, or if they realized they didn't have enough Stable space.

I don't really have anything to add on the actual suggestion being made (anything that would help newer players understand the function and benefits of releasing is a plus to me), however for the suggestion on a player level based selling feature, if you're trying to target newbies that way, it probably isn't going to work out the way you think...

Back when I started playing, end game was Astryl's Tranquility, so basically right before you jump into the Lost Grove now (level cap was 60). Leveling to 60 was, and is, very easy. Because I was having a lot of fun, I was level 60 and 'done' the story in a week. One week. I'd say I was still a newbie in pretty much every meaning of the word at that point. However, if there were 'newbie' pets set up for a certain level cut off, I wouldn't have been able to get them.

On the flip side, there are players who have no interest in the game's story and are here purely for the breeding. They could be here for years and years and only be about level 30-ish. Definitely not newbies, but able to buy and immediately release pets for whatever reason (points on release boards, points during the Patriotic event, to attempt to help the economy... whatever).

I do remember there being newbie giveaways and newbie sales in the forums that had a number of days/months on site cut off for sales. That is possibly a better way, however you also have people that sign up for the site, play a couple days, leave for five years, then return. Per their number of days shown on their profile, they're a very old player, but in actuality they're very much a newbie.

Very difficult to say who is 'new' and who isn't, unfortunately. Not to say further options for public sales would be bad, just saying that there isn't a good catch all for only selling to new players.

sadly the only thing i can suggest for this is to run private sales through forums for newbies.
i had the same issue, eventually i just stopped listing things for sale (with the exception of being over 15k) unless i was okay with them being released. now when i want to give something to someone, i do it through a private sale and set someone up with a link.