HM Customs & Excise often enter items for auction seized from smugglers, fraudsters and racketeers. Executors may enter the assets of individuals dying in testate (i.e. without leaving a will) or in debt. The official receiver handles the liquidations of companies that have failed. Bailiffs will enter goods into auctions which have been seized for various reasons such as non-payment of bills or failure to keep up payments to finance companies.
I wasn't aware that the TSA sold all of the stuff it confiscated! That's cheap goods right there! My mom works for a cruise line, and they don't confiscate stuff, but people do tend to forget all kinds of things they bring on the ship, so every once in a while the company has a big flea market available to employees. I find it funny when my mom comes home with watches and bracelets she wouldn't have otherwise bought.