I may be misunderstanding what you are asking, but the "Reply with Offer" to an ebay message as far as I know is only available on .COM at this time.
If someone has sent you an email and wants to negotiate, an alternative solution is to temporarily put best offer on the item with whatever you are willing to accept. Advise them to just put in an offer for that amount and it will be automagically accepted. If they don't end up buying it, you can revise the item to take off the best offer again.