Well you can sell it even though it is not legally yours, the buyer might me able to use it to pirate expressvu, it's not like actually cares what happens to expressvu receivers after they are desubbed. If they wanted to sub on it they would have to pay a rental fee, which defeats the point of buying it.
It would make more sense if they paid 15-20 to get it sent back, they could always rent it out or use it as a refrib.
I would call them one last time, and get the CSR's name and id#(and possibly record the conversation if you could, and tell the csr that it is being recorded) and tell them to send you a box to return the receiver or your throwing it in the garbage.
Knowing bell you might throw out the receiver tomorrow (without contacting them first)and the next day get a bill for $99 for a unreturned receiver, and if you refused to pay they would probley sell the balance eto a collection agency.