I know where to see expensive list pets and such, but I've been away for a while and I'm not sure where to see values by year for everything omgsc-very rare. Is there a forum post I could look at? Or could someone tell me any general trading guidelines and tips? Tysm! <3
The majority of pets don't have any particular 'special' value beyond what you see on their rarity bars. For trading any pet that us uncommon or below, you're most likely fine just matching rarities and possibly keeping years close depending on the user you're trading with. There's some exceptions, like the list pets you mentioned; because many of them are so old, they are difficult to obtain and therefore swapping 1:1 according to rarity bar may not be accepted or agreed upon by the larger community. Store pets are also typically valued higher than their rarity due to the fact that they will never be re-released; even the rarest monthly pets from 2008 have a chance to be obtained from the December 18th re-release, but store pets can only be obtained by trading once they are retired.
This post from Horror's list, a popular trading guide, goes over some rare and below pwts which are often valued higher due to how popular they are.