Personally I prefer not to compare item trading too closely to pet trading, obviously the basics are the same, but with pet trading we are often used to have very exact values for pets. Item trading is a lot more flexible and the fairness of items trades is a lot more subjective than in pet trades. What you consider fair may not be the same was what your trade partner considers fair. So I find it a lot easier simply focus more on making sure both sides of the trade are happy with what they are getting.
All that said, I think you'll find
this thread useful. It explains some of the details in what to consider when valuing items, as well as listing some (general) values for higher value items (some of which are out of date and in the process of being updated so again, general value). Users will also often post trades they have recently made with higher value items on the thread, to share what they paid for an item. So you can utilize the thread search to see if there are any recent trades for the item in question. Obviously just because someone else paid a certain amount that does mean thats what its worth, but it can help getting general values, or seeing trend (ie if most people paid a 2010 rare thats a safe bet on what to offer)
There is also
a thread for asking how much an item is worth, I'd suggest that too for any items you're unsure of.
Just in general for most regular items (like the free Easter items for example), I believe the generally accepted value is about a common for each, from the same year they were released. Sometimes more or less depending on the item, how popular it is, if people usually want a full set, etc. For example one item I can think of that would be valued less would be the 2016 gingerbread house building set. If you wanted the full set it would be far too much to expect to pay a common for each item simply because of the sheer number of items, and it was a free release, so its not particularly valuable.