There won't be any activation problems. When you have a retail license for Windows you have two options when you sell the computer:
1. Remove Windows from it, sell it without Windows installed, and keep your retail Windows license for use on another computer.
OR
2. Sell it with the Windows installed and provide the buyer with the retail license and product key for Windows.
It is not your problem that Microsoft has not provided a way to permanently deactivate Windows 10 on the computer you are selling and, therefore, if you choose to sell it bare with no Windows installed on it, it may affect your ability to use your Windows 7 product key in only one way - the next time you use your Windows 7 product key you might have to use phone activation and tell the computer voice that you have it installed on only one computer. And that would happen regardless of whether or not you upgraded the previous computer to Windows 10 because Microsoft has never provided a way to move the record of activation from their servers (all you can do is remove the product key from the local computer).
If the buyer installs Windows 10 on it and uses the old digital license to run it without their own license for Windows, it is the buyer that is responsible for that, not you. On Microsoft activation servers - the digital license for Windows 10 from an upgrade is tied only to the computer it was installed on - not to the Windows 7/8/8.1 license that it came from. It is only repeated use of the actual Windows 7/8/8.1 product key that gets it flagged to require phone in activation - not the repeated activation of Windows 10 with it's own digital license.