S3, E3: It's a bidding war on the Upper East Side. Blair and Chuck both set their sights on a particular item up for bid at Sotheby's, and the personal stakes are high. Plus, Serena suspects that Carter (recurring guest star SEBASTIAN STAN) may be up to his old habits, Georgina (recurring guest star MICHELLE TRACHTENBERG) takes an interest in Dan and Vanessa begins to suspect that something isn't quite right with Scott (recurring guest star CHRIS RIGGI - "Lipstick Jungle," "Dare").