I always keep the
Guide Index bookmarked - there's a ton of helpful resources there!
The Understanding Rarities Guide is a good place to start if you want a basic understanding of the rarity system
The "In Depth" Guide to Trading is a guide I'd recommend checking out if you want a better understanding of how the trade system works! Everything you'd ever want to know about trading and rarity can be found here
Other threads that can be helpful when you're making a trade you're unsure about are
"Is this a fair Trade?" Thread and
"How much is this pet worth?" thread. There are more threads such as these where you can ask for advice on things like C$ values and items if you're interested - all of these can be found in the Guide Index
hope this helps! c: