May I sell a copy of windows 7 after installing and then removing it?

You need to check your user license. You probably can not legally resell it.

if it is an OEM version, then nope.
If you went to the store and bought it, I see no reason.
If the license key shows up on 2 computers at the same time, you will certain know as it will get flagged after being connected to the internet.

If it’s an OEM version then you can only sell it with the computer.

If it’s a retail boxed version you can as long you unistall it.

If it’s Windows Anytime Upgrade Software version (even boxed) you can sell it but only with the computer it was licenced for. You also can’t keep any copies of the earlier version.

@blueiiznh @Lightlyseared it is a retail version and I am not using it, I’ll sell it. Thanks!