Basically, if you have one successful trade, that means that you do get your cards at some point in time, you give the person you traded with 1 point, but saying:
There name +1
If you do not receive your cards, ever, you give them a negative 1 in the same manner:
There name -1
On one of the pages, there is a list with all the users names on it, and a number next to them. The higher the number the more trustworthy of a trader they are. Right now, I think I have 1 point, that means I have made one trade and it was successful, we both received our cards. Those who have a negative number only next to them, are untrustworthy, as those who have traded with them, never got there cards.
I just received my card from plmrelm actually, so I give him, +1.
Also, plmrelm +1