sim card

Yes, I would do it. Why should you leave it inside that iPhone?

AND make sure you follow all the steps provided here:--> What to do before you sell, give away, or trade in your iPhone, iPad, or iPod touch - Apple Support

Regards

Giulio

You are most welcome :)

Are you going to change carrier and/or phone number?

Shouldn't be this case, that SIM card could be okay for your new iPhone (it's a nanoSIM, compatible with the vast majority of iPhone's models)

Should be this case, in any case you need to stop that SIM card activity informing your carrier.

So, why should you leave it in your device before selling it?

Regards

Giulio

It really doesn't matter.

If you want, you can but it really does not matter.