Assuming you mean send an offer to buyers who might be interested in buying your items (as opposed to you sending an offer for something you want to buy), on the left side of your Seller Hub page, under the heading "Tasks", there is an item that says "listings that are eligible to send offers". If you click on that, it will bring up a list of things people have on their watch list and you can send offers that way, either a specific dollar amount or a percentage off. You don't know who you are sending the offer to, just that someone is interested and you are offering them a better price.
Also, just because you send someone an offer doesn't mean they will buy the item. There are many reasons people have items on their watch list.
