The used market has been heavily hit with the increase in demand for bikes due to quarantine. There may be an uptick in supply when all this is over and non riders sell their bikes, but expect to wait for awhile for the right used bike to come along.
In regards to a build, do you already have some of the components? I don't see you staying under $800 without already having some of the parts (wheels, drivetrain, etc...). Also, look for 135mm (QR) or 142 (TA) rear spacing if buying a frame. Thru axle has taken over most of the market.