Trade cost
Trade are grouped as follows:
You can trade 20 times the same item.
It gets expensive when creating multiple trade, so wait till you no longer have an active trade in the 'your trades' tab.
This is about active simultaneous trade, not the total of trades you have ever made.
First trade: Free
Second: 100
Third: 300
Fourth: 700
Fifth: 1500
Sixth: 3100
Seventh: 6300
Eight: 12700
Ninth: 25500
Tenth: 51100
Eleventh: 102300
Twelth: 204700
Thirteenth: 409500
Fourteenth: 819100
Fifteenth: 1638300