I have sold lots(20 or so) of 3rd hand bikes- mostly a hobby
almost all were MTBs -they are good do everything urban/suburban/trail bikes(nice wide tires)
The buyers all wanted a good price decent bike with working brakes shifters
Good price means price relative to equivalent new bike-ball park 20-25% of new
and do the brakes and shifters work were the main concerns
Price and condition sells used "stuff"
but I can see someone wanting a "fast looking" or cool looking or light bike
but MTBs usually aren't fast cool of light so that usually didn't come up