At times, customs may decide to not only seize goods but detain the passenger as well. This can happen for a number of different reasons. One reason is if, as mentioned above, a passenger has not declared the item(s) or if they have been declared falsely. Another reason would be if an individual carried with them over 200 grams of tobacco or a very large amount of alcohol.
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.