What is Tier trading, explanation please?

[kij0]

Hi!
I'm looking for someone who will help explain the whole 'tier trading' deal to me.
I have a UR/OMGSR Pheonix (from December 18th) and I need to find out its true worth. I've already gotten two offers I don't want to get scammed.
Yes, I have looked at the *DISCONTINUED* Rares List Guide (1 March 2019) thread, but I can't make any sense of it.
Whoever is available to help me, please respond to this post or PM me so we can talk.
Thank you!

[Mid-life]

.
Hello! This thread and this thread are super helpful resources! According to the second thread I linked, the UR Phoenix is worth 1-2 nons. However, worth is always up to you, so if you want to trade it for something higher or lower it is your choice, as long as you find the trade fair / like the pets you are recieving.