The reply with offer function can be handy, but it's very limited unfortunately.
It's only available in single item listings, so won't work (or appear) when a buyer messages about a listing that has variations, and you can only use it once as well (I had a buyer that I sent an offer to with it, and a little while later they replied to the original message asking if they could get the same deal again, I didn't have the reply with offer button so I asked them to send me a completely new message via the listing....still didn't get it. So then I went back to the original message which did still have the button, but when I tried to send through the offer, I got an error message saying the buyer had exceeded their limit of offers).
So now I usually just stick to creating custom listings, just for consistency really (i.e. saves having to explain I can only send one offer, ever, then clarifying that I can still provide the price, just not via that function).
