Hi manic_34,
If the seller is unable to remove the iCloud Lock (in this case, the lock imposed by Find My Mac), you should assume that the device is stolen. DO NOT PURCHASE THIS MAC.
Apple and their authorized service providers have the capability to remove any lock imposed by iCloud, but will only do so if the person taking the Mac in can provide proof of ownership (such as an original receipt).
EDIT: The safest place to purchase used Macs from is the Refurbished Products section in the Apple store: Certified Refurbished Products - Apple