How do I offer watchers a discount price?

I have noticed that when you watch an item some sellers offer you a discount. How do they do this?

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Contrary to what was posted earlier on in this thread, you do not have to be signed up to receive such offers in order to be able to receive them.  I have never signed up to receive offers from sellers and would actually prefer not to receive them - if I really wanted an item badly enough then I would either make sure that I won it via auction if it had been listed that way, or I would pay for it via Buy It Now if it was advertised via a fixed price listing.   However, I have, on quite a few occasions, recieved offers on items that I have previously looked at, none of which I have even decided to add to my Watch List.  Therefore, the so-called "offer from seller" is really nothing more than an eBay-generated piece of spam mail.