I have two of these, either or both are available, $35 each, cash only. I have been the only owner in a smoke free home. They are in great condition.
The primary use for these is to provide wireless connection for rear right and left channels in a surround speaker setup. One unit will connect two speakers. It works by having a small powered hub nearby the AV receiver and which has the two speakers channels wired (w/ speaker wire) from the AVR to the Rocketfish hub. It then sends a wireless signal to its companion hub (wall powered) located elsewhere in the room, which has the two actual speakers wired to it.
These have always been extremely reliable, apparently tolerating a good bit of furniture in the way without affecting performance. I have never noticed any lag. The only maintenance is if your house loses power you have to push a button to reconnect. They have one job to do and do it as headache-free as one could hope.