OnlySheets + Gumroad
You're purchasing OnlySheets Integration with Gumroad.
Check your email for the link to the sheet.
Sell access to any google sheet via Gumroad.
No need to set your permission level on a sheet to "Anyone with a link"
Your sheet, Your Choice.
A google script will handle adding the buyer as a viewer.
You can also sell Edit access, if you wish.
A google Sheet Template and code.