If you find debris, the first thing you should do is call the local authorities. The next thing you should do is, protect the debris. The last thing we want is the memory of the Columbia Seven to be desecrated by sales on Ebay. In my humble opinion, all debris of Columbia should be imediately MOVED to safe locations under trusted guard until it can be picked up.
NASA will need every possible scrap for forensic study, so that they can determine the cause of the tragedy and prevent it from happening again. Sadly we do not have Richard Feynman to analyse the data, but we can at least do our part by making sure that the ghouls don't get their hands on the physical evidence. Save what you can and call the police.
UPDATE: ok, NASA clarified by saying that the toxic nature of the propellants are what makes debris dangerous. So, be careful! But don't be afraid of radioactivity - and it is still vitally important that we keep as much of the debris out of the hands of scavenger ghouls as possible.
UPDATE: It already started, and will likely continue:
Risk said a group of five to six people were watching over what appeared to a PC board found in the passing lane in the intersection of Martinsville Road and University Drive when a woman in a 2000 blue Volkswagen stopped, jumped out of her car wearing a while plastic bag over her hand, scooped up the debris and began walking away.
"We asked her, 'What are you doing?' and she said, 'None of your business,' got back in her care and drove off," he said. A person in the group took a photo of the car as it left, including the license plate, and notified authorities, he said. "We flagged down a police car, and they ran the information, found out who she was and where she lived," Risk said. "I hope they pursue it."
And there was already an auction for "space shuttle debris" on EBay, but it was quickly pulled (mirror). But there will be more.