A few forums on FB usually doing trades, but watch out because there's a lot of people out there conning people - someone in a group traded a Borussia Dortmund shirt for a PAOK one, only to find out that the PAOK guy never sent the Borussia one back over and left the group. There's a trade only group somewhere on FB, where they verify you before you can trade, but... it's still not safe.