Used cars are a commodity, prices can and do go up and down.
It would stand to reason if driving is down, and new car sales are down, the supply of used cars would also be down. You only get a used car when someone trades it in or if Grandma dies before going to the long term care facility.
Just for fun, went on local domestic dealership site. No 2021s coming in yet, yet have more 2020s on order for delivery. Between COVID and supply chain interruption new car production in way down.
Just a guess on my part, have not researched it.
Chevy dealer has used trucks filling in where the new inventory normally would be.