You can first look what the items are worth in pets, I know the Eros is very valuable ( goes for more then listed with demand of course that’s subjective ) and most of the items on the bottom trade go for an 09 rare/very rare like the French hens since they are older, I’ve attached the link to the items rares list, most other items go for a common to uncommon of the year they were released, except older, event and store items they vary . So once you find out what the items are worth in pets the use the how much is this pet worth in c$ to convert .you can always check with the how much is this item worth thread as well.
Items guide
Forum/viewtopic.php?f=20&t=559754How much is this pet worth in c$ Thread
Forum/viewtopic.php?f=20&t=3915876How much is this item worth
Forum/viewtopic.php?f=20&t=3719854Hope this helps a bit!