The best (worst?) case of this I ever 
experienced was when someone offered me a 
whopping $25 for a cel I paid $1500+ for. After 
I finished laughing, I politely e-mailed the 
person back and explained that there wasn't a 
single cel in that particular part of my gallery 
that had cost under $30, probably, let alone 
this particular cel, which is an eyecatch.

My lone bit of sarcasm that I let slip was "You 
don't actually collect cels, do you?" 
Restrained, on my part.

And yep, I did feel insulted on behalf of my 
cel. Heh.