When you have something to sell, you can contact a local newspaper, but that means that only locals will see it, and you have to pay to run your advertisement. On the other hand, posting to an online classified site will allow locals to see your ad, but it might also allow people all over the country or world see it. Furthermore, many online sites will run your ad for free. Before you just post your ad willy-nilly, you should choose a site that will get the exposure that you are expecting.
Some sites will specialize in certain types of classifieds. For example, some sites will only sell tickets. This does not necessarily mean that you can't post ads for other items; it just means that people who visit the site will be looking for tickets, and if they come across your ad at all, they likely will have no interest in it. On the other hand, if you do have tickets to sell, a property to rent, a car to sell, etc., finding a specialized website is the best way to draw attention to an item that would otherwise be buried in a miscellaneous site.
Each site will have its own guidelines for what you can say in the ad, what items or services you can sell, etc. If you post an ad that does not meet the guidelines of the website, they may flag your ad or take it down. Take the time to study the site guidelines so that you don't run into problems.
Making a sell to someone who lives across the country or across the world can be complicated. If you have a valuable item or a motivated buyer, then maybe it is worth it to you to post on a site that will distribute your ad around the world. On the other hand, many of the items you sell on a classified ad site will not be worth the hassle. Thus, you will want to check whether the site you choose publishes its ads all over the world, or whether they focus on local distribution and choose a site that matches your selling needs.
An online classified site can be a great way to sell items that you no longer have a need for. You just need to make sure that you are using a site that will get you the sale that you are after. Classified sites will try to target different sectors of people. Make sure the site you choose is designed with your needs in mind. Click here for more information on free classified listing.