Craigslist.
Market it carefully with a complete description (Colors, options, mods, services, condition descriptions, reason for selling) , make sure it is detailed to an almost professional level, understand why your car is the one they should have and market those distinctions. And price.
Make the car be made attractive by the obvious care the seller took in preparing it for sale and the information provided.
Understand fall is here and it isn't like selling a convertible in the spring.
Understand the Boxster is not collectible and is a 10 year old car worth only what someone will give you for it, it has no extra value because you loved it.
More Thoughts
good luck.