Well, I haven't personally had an extremely low 
offer made...  but I've made a few myself which 
turned out to be significantly lower than the 
cel's owner was looking for.  Though in at least 
one case, I was offering her the price I'd seen 
on a cel from the same sequence on a dealer's 
site, so I didn't feel guilty about that.

In both cases, the seller told me that they were 
looking for more and then named an amount they 
found acceptable.  They were polite about it, and 
I bought the cel for that amount in both cases.

I'm wouldn't be insulted by a low bid, but I 
would reply naming the price I'd take for the 
cel.  Though, if you're not selling in the first 
place, it's a moot point.