My opinion on this is that because cels from the 
same series and even of the same charecter can 
differ so much in price that maybe the person 
offering did not realize the worth of the cel.
I doubt they meant in offense to the value of 
your cel.
Also the prices of cels have been down 
considerably so this may be a new collector who 
is basing thier offer on the cels that they have 
sold recently. The person also has no way to tell 
what you paid for a cel so they had to base their 
price on a guess or what they would pay.

I personally HATE making offers for fear of the 
price being to high or to low. I don't think that 
your husband should have been rude in an email, 
he should have just stated that he didn't believe 
it was a fair price and either replied what he 
wanted or simply that the offer was declined.

What I would do in this case would be to write 
that person and explain that you or your husband 
didn't mean to sound harsh but that you were 
reacting to what you percieved as an insult. The 
person you offend now might be the person that 
has your dream cel one day and it is better not 
to burn bridges.

this is just my opinion 
Julie