I just found this group today. My son and I have a pretty decent collection of cards and we're pretty excited about this.

I couldn't find a general information page that might help us to understand what fair trades might look like. For example, would you trade four commons for a rare? We have no idea and have never actually traded cards before.

Also, how do we know what collections the cards that we do have belong to, so we can begin to add our cards here.

Hello,

First of all, welcome to Pokéchange !

For example, would you trade four commons for a rare? We have no idea and have never actually traded cards before.

It depends, but in order to give you an idea, some people apply the following equivalences :

● = 1 pt
♦ = 2 pts
●r = 3 pts
♦r = 4 pts
★ = 6 pts
★r = 10 pts
★H = 12 pts
★Hr = 16 pts

Nevertheless, some people also trade cards in order to have other cards of the same rarety, and in the same state/condition :

1 EX = 1 EX, 1 GX = 1GX, 1 trainer FA = 1 trainer FA, 1 secret rare = 1 secret rare of the same expansion.

Also, how do we know what collections the cards that we do have belong to, so we can begin to add our cards here.

You can identify your cards' expansion  by looking for an icon that is visible on the bottom-right or bottom-left corner of your card.

Here is an example : https://www.pokechange.net/uploads/support_24.png

Thanks for your quick and concise reply! 
Is there a FAQ page somewhere here? The website seems mostly intuitive but not entirely. 😀

You can find some help videos here (the interface has changed a little bit since these videos were made) :

https://en.pokechange.net/help

I wish you'll be able to do great trades

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