Two simple things might be the cause - a dirty lens or the transport lock. If you can access the lens give it a wipe with a cotton bud dipped in isopropyl alcohol. Make sure that the player is switched off first, don't forget that it's a laser and can damage eyesight!
The transport lock is normally a screw or switch on the underside of the unit. Maybe the seller flicked this before you collected it?
Otherwise, there are mutiple reasons, such as:
Disc loaded upside-down.
Dirty, scratched, or defective disc.
Dirty or damaged objective lens.
Loading (mechanical) not completed reliably.
Damaged lens suspension or damaged lens cover preventing free movement.
Dirt, gummed up lubrication, or damage in sled drive mechanism.
Dirty/defective limit switch or sensor.
Defective spindle motor.
Spindle table height incorrectly set.
Bad component in optical pickup.
Cracks in ribbon cable to optical pickup.
Need to adjust servo (or less likely, optical) alignment.
Faulty power supply, electronics, or control logic.
Bad connections including missing/erratic optical deck shield.
External interference.
Here's a link for more information
http://www.repairfaq.org/sam/cdfaq.htm#cdnokcad