Edmunds lists a Boxster RS60 with 8k at $39,907 outstanding retail, and $37,462 private party.
NADA has a regular 2008 Boxster S with 8k miles at almost $40k at clean retail. Mid-thirties 3rd party/trade-in. Not sure what the bump for an RS60 would be. But presumably something.
This dealer is asking $53k for the same car with 12k miles:
2008 Used Porsche Boxster RS 60 Spyder at CNC Motors Inc. Serving Ontario, CA, IID 9678836
This dealer is asking $34,577 for a 2008 RS60 with almost 38k miles:
http://www.napletonsastonmartinofchicago.com/used/Porsche/2008-Porsche-Boxster-0dd5d70e0a0a00023fafc743abc80399.htm
$40k sounds like a retail price, but doesn't sound out of the ballpark if it is as well-kept as described. These don't really grow on trees, so if you want on, and it is local, I don't know if a couple of grand would be a deal breaker.
Maybe he will knock off a grand to close the deal since ya'll don't seem very far apart.