el capitan installer asks for apple id

You must restore the Mac to original version of macOS it shipped with. If you don't do that it will be tied to your Apple ID permanently and the new owner will be unable to upgrade it.

The sambas true of any application you have installed. They also are permanently tied to your Apple ID so you should uninstall them.

Then you wont be installing it. ElCapitan is not yours to sell, the Mac must be sold with its original OS, whatever that was.

Yes.

Yes.