It stands for primary. On modern bikes, it's OK to leave it in this position, because it won't leak. On older bikes such as yours, it should be left OFF when not riding. It's not so much it'll damage anything, but you could drain the tank.
If you like the car, I'd buy it. Grand Prix's are cool. But first check the value on KBB. They will be shutting down Pontiac. But I wouldn't worry about getting it serviced because the Pontiacs share 90% of their parts all with other GM brands.
Hmm. I wonder what the definition of probation is. Oh, here it is:
A process or period in which a person's fitness, as for work or membership in a social group, is tested.