As of right now, there is no item value list or website that is officially made or endorsed by Adopt Me. This is because we leave it up to our community to decide what they believe each item is worth and determine what items are in demand. We do not wish to impose on that.
However, if you're lost and unsure what your items are worth, we'd recommend heading to our official Discord server and checking out our dedicated trade discussions channel and our "Win/Fair/Lose" channel to find out more about your items and trades.
To join our Discord server you must be over the age of 13 as per Discord's Terms of Service.