This article is from the Macintosh for sale FAQ, by Elliotte Rusty Harold firstname.lastname@example.org with numerous contributions by others.
Make your lowest estimate of the resale value of your hardware
taking into account current prices on new hardware of equivalent
capability. Then divide that estimate by two. The price you now
have is somewhere above the fair market value of your hardware.
Almost everyone severely overestimates the value of their
equipment. I've seen friendships and business relationships
destroyed once buyers realized how badly they'd been taken EVEN
THOUGH THE SELLERS DID NOT INTENTIONALLY MISLEAD THE BUYERS.
Computers are like cars in that they lose 30% of their value
as soon as you walk out of the dealer's showroom with one. They
are unlike cars in that they lose more value when new models are
released irregardless of the condition or age of the previous model.
Unlike cars newer models of computer really are better. High-end
systems retain their value longer. My 8/45 SE/30, a high-end system
when I bought it four years ago, is still barely salable today.
Had I purchased a low-end Plus instead it would at most be worth
$100 for parts.