The 3rd gen Echo Dot remains the best of all Dots. Might even be the best all around value Echo device regardless of model or generation. Same sound quality as 4th Gen Echo Dot but does not have that silly globular dust-collector fabric form factor and does not hide its LED status ring underneath like the 4th Gen Dot.
$20 for a less than “Very Good” or “Like New” seems a bit high in that these were regularly available new for $20.
FYI, these things love responding to Amazon’s own TV commercials. 
Actually, no, they don’t. Never have at our home - and we have them in every room with a TV.
Amazon has inaudible to human ear audio encoding in their ads that signals Echo devices to ignore their ads.
really? After around the 12th time one evening, lots of amazon ads back at xmas, mine was unplugged and banished to the garage, never to return.
Yes, really. We’ve watched Amazon ads for Echo devices with Echo devices in the room and I don’t recall hearing one respond. We’ve been using Echo devices since the 2nd Gen Echo Dot was released - end of 2016(?). (Strive to avoid new technology until the 2nd Gen.)
However, we have had them (rarely) respond to third party ads or “Alexa” in programs. From producers who didn’t bother to add the “ignore” content to their media.
However, watch any Star Trek and it wakes up continually when the wake word is changed to “computer”. Yes it’s my choice but nevertheless that is a thing.