He puts the card or cards in your ps
and you put the cards in his ps
if there is an amount you both agreed on
you or he puts the card for that amount you guys agreed on in your or his ps
if there is no agreed amount you and him put the cards for 50 clintz in your or his ps
if any deal is made you can just private sales them ( in case if they do cheat with you ) contact customer support -- > https://www.urban-rivals.com/staff/contactus.php
and you get your cards back and the player who did cheat with you gets a ban as well, easy
I will add to what Wowed Supremes and Crimer has mentioned and address your error message.
Select "Private Sales"
For Trade, select a card, enter your price, the name of the player (it must be spelt correctly to send).
In return, if you check your private sales, the card should be placed there.
The minimum for PS is 50 Clintz.
When contacting Customer Support, a screenshot of the thread and your sales history can be used to identify cheating in trades and strengthen your case.
Yes, there is a blacklist for players who do not honour any deal where there is a mutual agreement.
For the error message, I am not sure if you are using the App or site. Trading with another player should not generate the error if you select "Private Sales"