I am using Gumroad to sell digital assets currently. This is a popular and well known platform among creatives to get their work to a broader audience.
Please note that you will be redirected to the Gumroad site in order to proces payment in a secure manner. Another one is the Unity Assetstore where you can buy assets directly in Unity.
Best regards! Tom
Click here to visit the Unity assetstore with my assets preselected.