I am a used Porsche dealer. This is what I recommend.
Do not buy any car over the Internet without inspecting it yourself.
Have the car inspected PRIOR to purchase by a Porsche dealer. A full inspection will cost about $200 and run from the bottom of the tires to the top of the convt top.
Run a Car Fax.
Make sure the dealer performs a check with a paint meter.
Have the dealer run a warranty claim history on the car.
Ask the seller for any service records they may have.
This is a good start.
Good luck.
PS-no one knows the value of your car into the future. Our guess is as good as yours.
