Hello
You don't have to tell them why, but a simple "No,
thanks" is sufficient for a reply.
I recently went to some effort to come up with
prices for a list of cels that someone requested,
most of which were cels I didn't really want to
part with, but hey - I'm willing to be reasonable.
I admitted up front that I realized I'd overpaid
for some of them. Now, I went to some time and
effort to reply to this person who didn't even
make offers on the cels and have heard zippo back
from them about it. 
Ahwell, but if you send a mail to someone making
an offer for something in their gallery and don't
get a response... maybe they're like me and bloody
tired of spending time dealing with people who are
not serious  about conducting a transaction.