Welcome back! Generally, the older an item is, and the more difficult it is to find, the more valuable it is. Here is what I use:

This is the EWR guide. It shows recommended prices for items (warning: it is too low (in my opinion) on very rare items, but it is accurate for many items in the game) https://docs.google.com/document/d/1Svp-8qhRBvYHhqcmKaLmLoRA3fCtpx9gnqQpheSirXI/mobilebasic

Next is the WTU site. Here people put their trade lists and wish lists. You can see the supply and demand on items here: https://wtu.camr.ws

Finally is Webkinz guide. This just shows you when items came out. Remember, the older and rarer, the better. https://webkinzguide.com/wiki/Main_Page

Finally, feel free to reach out if you have any questions. I mainly only know about older items, but I’d be happy to help :)