When you find that you have purchased all of your properties in GTA Online, it might be time to move on to something new. You might find somewhere bigger, fancier, or even closer to your favorite part of town. You have to take a unique approach to selling a property on GTA Online because it is not easy.
Before you can sell a property on GTA Online, you need to fill in all of the currently available slots. You can't do it any other way. It's a small little detail that can easily trip you up if you want to sell a property that you have, but you don't have all of your locations occupied. You can choose a smaller location, even if you don't plan to use it to help fill it.
When you complete all of your property slots in GTA Online, you want to go to the website to buy a location ready to pick your next pick. When you find the next property you want to enter the game requires you to choose one of your existing locations to sell on the open market. Just like in the game, you will not receive the full price you had to pay to purchase it. You'll want to choose the one you originally wanted to remove.
After making your choice, you will receive the money from the sale of the place and have the chance to buy the new one. It's a strange system if you want to sell a location, and forces you to keep your slots maximized, but understanding how this strange system works is extremely important.