I am going to guess is V6 Syclone motor not a Mercury Cyclone V8
The balancer sometimes spins especially with higher mileage, so you need to do a TDC check with a piston stop. Once you have established the TDC mark on balancer, then set the timing to 0 degrees.
Before your pruchase I would becareful looking over the motor, if its stock internal and he knows it has timing issues from audible pinging, it maybe too late for the lower end(broken ring, melted piston or bent connecting rod)