There is an option to buy a Seller's products at a pre-set price that that Seller has defined. This is called "Buy Out". The Seller has to elect to offer this option on that specific product, and not all Sellers will opt to do so. If a product is available for you to "Buy Out", you will see this option available on the product's page. This feature saves you the time of having to watch an item and out-bid other Buyers.
As a Seller, you can flag your product as available for "Buy Out" in Step 4 ("Settings") of the submitting an item for sale. Simply check off "Enable Buy Out" and set the "Buy Out" price for the product.