The Post Gazette has a good article revealing just how the selection process for the show works. It seems that they don't exactly just let anyone just bring things in--until they have screened things first online.
"Prospective participants were solicited through the show's website (pbs.org/wgbh/roadshow), where photos of items were submitted for pre-approval. That's no guarantee that Rich, a warehouse manager in Canton, Ohio, or Bob, a long-time antiques collector from Beaver County, will actually make the final cut with their treasures. It does mean the show is interested enough to arrange for the items' delivery to the convention center.
Not long ago, Rich -- the show won't allow last names, insisting on a certain level of anonymity for its participants because of the possible value of the items -- was hoping to sell his garish but fun Tiki bar for $150."
P.S. All 6000 available free tickets to the event were already distributed by lottery and none will be available at the door.