A used engine can be bought for $3000 and swapped in for an additional $1500. Total cost to get the car back on the road around $5K. (
2002 Porsche Boxster 3.2L Engine $3000)
A Boxster rolling chassis is typically worth $3K-$4K. Yours might be worth a bit more but its hard to imagine a scenario where someone pays over $5K.
The primary reason for the low price of a rolling chassis is that there is a lot of risk in purchasing a car that doesn't run because there are a lot of components that can't be tested (like air conditioning, engine cooling, clutch, power steering, CEL's for O2 sensors/SAI/fuel system, etc, etc). Thus, the buyer has to take that risk into account and factor it into the purchase price.
And while you are likely a very honest person, this is a very real risk. In my personal experience, the Boxster that I bought with a failed engine cost me another $1,000 to fix things that the seller "promised and assured" me were in good working order. Lesson learned.