As a bike shop owner, the main obstacle to used bike sales is finding people with good quality, well cared for bikes that they don’t want any more and are willing to let go of for half what they would fetch on the internet.
Oh, and with proof of ownership too.
Occasionally I get lucky, but not often enough for used sales to be anything more than a bonus every now and then.
I’ve got a regular customer who buys job lots from the police unclaimed stocks. He’ll maybe get one quality bike for every 50 well battered BSOs.