I myself bought a 2nd hand 4 yr old car. IMO i prefer to buy it
directly from a pte seller rather than from a dealer. Unless the
car is too rare until it can only be found from dealers.
Since I've only bought cars from Pte owners, i can only give
these tips,
Check OMV,current paper value.
Find out how much is the seller selling above paper
If possible, request for the seller to bring the car to YOUR
trusted workshop/Mech for a Full day check up for any major
accidents,defects, Modifications etc.
Request to change all Timing belts,air-con belts, water
pump,bearings or anything which needed replacing after the 3rd
yr.
Proof of servicing history during the ownership of the vehicle
if possible.