It is "worth" whatever you will pay, and the other guy will accept.  I
usually won't price someone's merchandise until they price it themselves.
Then if that seems too high, I will either offer less, or simply say "no."
If it seems right, I buy it.  I doubt that he is offering any guarantee,
other than that it is what it claims to be.  If true, that would make it
'worth" less than a quality rebuilt from a reputable vendor to me.  After
all, brand new stock is sometimes bad.  If it has been on the shelf a long
while, who knows?

Just my thoughts.  mcneely