A public notice promoting the sale of a certain item is called an advertisement. The word is often shortened to ad. They're on TV, on the radio, online, in the paper — you can't even escape advertisements at the movies.
Although advertisements are usually meant to persuade, they aren't always just for selling goods. They're a great way to get the word out about just about anything. You can place an advertisement to (among other things) look for your long-lost sister, hire an employee, or announce a local dance. Even way back in the 1600s, the British were posting advertisements for trips to a newfound place called "Virginia."