Speaker Invoice Via Audiogon

Yes. You need to accept his offer, (or if it was a Buy It Now). Then add the shipping cost, (even if it is zero). This will close the sale and will send an invoice to the buyer to pay from.

In case the buyer is expecting something else.

I usually include a screen print of the "For Sale" ad that I used showing the item as "SOLD". Put it in the box when you ship. I keep a copy for my records as well, in a binder.