It is essential that you establish a location on your profile (You > User name) in order to sell on Wallapop, because products are ordered by proximity.
To do this, access your personal area where you'll find the option to add your location below the number of purchases and sales. You'll see that your profile is without a location and you have the option to add one, by entering your address or post code.
The application will link your profile to the indicated area and nearby users will see your products for sale.