Do I need to include accessories when trading in my iMac?

No, you don't need to return the accessories. Just the computer and power cord.

How to prepare your iMac for Apple Trade In - Apple Support

"(Do not include extra items or accessories. If you include extra items in the box, we won't be able to return them to you.)"

Also, prior to shipping it in document with photos that show the date and time stamp that the computer is running perfectly. Also shoot photos of the chassis and screen to prove there is no damage prior to it being shipped. This is important as we have seen reports on these forums of trade-in quotes being changed because the computer was not in the shape the owner claimed it was when they sent it in, so do your due diligence and prove it's in perfect shape prior to shipping it in!!!!!!