If I am trading in an old iMac, do I need to include the keyboard and mouse?

Depending upon the age, peripherals may get you slightly more. If it were me I would keep them as backups, or simply because I prefer the older style keyboards.

Apple Trade In. click on the link --> https://www.apple.com/shop/trade-in Also read the FAQ at the very bottom (it discusses this kind of thing)

You are free to keep the keyboard and mouse. Including accessories is entirely optional.