You will get notified through email when you have a new sale on Artboost. In this email you will be directed to your order-page. On the order-page, which you can also access directly from your Artboost profile, you can reject or accept your sales. In case your artwork is already sold, you should reject the sale.
To avoid selling artworks that you’ve already sold, you have the ability to mark your items as “sold”. This is done inside the edit tool on specific artworks.