We've had a few questions come in about legitimate places to buy, sell or trade U2 concert tickets with other fans, so consider this a Public Service Announcement aimed at answering a question that may be on a lot of fans' minds.
Officially, there are still some tickets for certain shows available via official ticket outlets (i.e., Ticketmaster in North America). The tour page on U2.com appears to be pretty up-to-date about which shows still have tickets available.
Beyond that, we also have a dedicated board in our forum for fans to buy, sell and trade tickets directly with other fans and only at face value. All of the existing shows are listed individually, so just find the show(s) you're interested in and add your post or see what others have already posted. Selling or trading GA tickets may be difficult due to the paperless ticket requirements, so keep that in mind as you offer those tickets or search for them.
And be sure, if you do use this section of our forum, to read the post at the top of the board -- "Rules for Ticket Buying/Selling/Trading." We don't hesitate to boot people that try to make a profit from selling tickets in our forum.