Do you have a lot of stuff lying around that you really don't want or need? You know the famous saying "One man's trash is another man's treasure"? Well this may be the case for you. If you are looking for some extra money and to get rid of some of your things, then you may want to consider selling them in an online marketplace.
Look around your house for smaller items that you want to get rid of. Items should ideally be no larger than a printer or computer, if they are then shipping becomes more complicated and expensive. I recommend having a day where you sort through all of your things to find what you want to sell, not only will this help to find things to sell, it will also clean up your house a little! Make a pile of all the things that you are looking to sell.
Make a list of all the items that you wish to sell. Determine the smallest amount that you wish to receive and the estimated shipping cost of the item (the easiest way is to go to the website of the service you wish to ship with and use the price calculator).
Go to the site of whichever online marketplace you wish to sell on. Make an account (if you do not already have one) or sign in (if you have an existing account).
Appropriately price the item and the shipping cost. Buyers may get frustrated if the shipping fee is too high. Don't expect to get paid what you paid for the item. These kinds of online marketplaces are mostly represented by buyers looking to get a discount. It's probably a good idea to search around the site that you are selling on and see what the value of your item is.
Things to Keep in Mind:
This isn't a get-rich-quick scheme. You will likely make some extra money, but you shouldn't expect to make tons of money.
Be kind and polite to buyers, if you are, then they are more likely to buy from you again. If you are rude then your reputation will go downhill and they buyer won't buy from you again.
Shipping may be more than you think. Make sure you do your research.
Find the right marketplace for the item. Some sites are geared more towards smaller items, while others are geared towards larger items.