Someone ending an engagement or marriage can sell or auction off his or her your engagement ring and other jewelry on IDoNowIDont.com. Mara Opperman, cofounder of the site, says its average transaction price is $2,500, usually 30 to 60 percent below retail, so it can also be a place for those planning an engagement to snag a deal on a ring. The site is full of interesting back stories detailing relationships gone bad.