I could see where you got insulted ;)  I'm sure 
its' happened to all of us.  I generally don't 
make offers on cels because I NEVER know what to 
offer.  Its' a risky thing.  Over too low, and 
you'll insult someone.  Offer to high and you 
could get screwed when you figure out you just 
offered $200 and they got it really cheap.  I 
generally just purchase cels that actually have 
price tags on them.

I also try not to ask cel collectors if they 
would be willing to sell because alot of people 
will get really snotty.  "Sorry this LOVELY cel 
is NOT for sale."  Almost implying "You're not 
even good enough to LOOK at this cel how dare 
you ask me such a question."

Alot of series have dropped in price 
dramatically, and the person might be a newbie 
who frequents ebay auctions where bids are as 
low as $40 for a good cel.  So whereas $60 
sounds reasonable to them its not what you're 
looking for.  I always just tell people 
politely "I'm sorry but I can't let it go for 
that.  Thank you for your interest though."  
Then its up to them if they want to offer more.

The way I see it is if someone asks politely "I 
know you aren't selling any of your cels but if 
you ever have to and you'd be willing to part 
with THIS one please let me know." As long as 
someone asks politely its' cool :)