There's probably some law that force them to sell any confiscated goods. Just like they sell other stuff, they have to sell Bitcoin too, I guess.
So are they also selling seized drugs?
I don't know about them selling seize drugs. The Gov't sells their own illegal cocaine to children, and they do not want competition cutting into their profits. Please read http://www.infowars.com/drug-war-hypocrisy-drug-traffickings-big-money-benefits-big-brother-and-corrupt-banksters/ it is solid factual reporting.