When you are in item's detail page and the item's price is listed with negotiable status then you will see two buttons "Buy" and "Offer" in front of "Purchase Qty" field.
If item's price is only negotiable then only "Offer" will appear. However if the item's price is fixed then "Buy" label will appear. Buy refers to buying something while offer refers negotiating your specified offer.
When both seller and buyer get agreed (i.e. both agree on one price) after that they can use action "note" to send messages to each other.