My prior iMac has no trade-in value but Apple describes the steps to clean the system prior to returning it to Apple for safe disposal.
iMac 24″, macOS 14.5
... and dialabrain just provided you with a link to an Apple Support article that takes you through the steps to prepare your old Mac to be disposed by Apple.
Here it is again: What to do before you sell, give away, trade in, or recycle your Mac - Apple Support
I have a new 2023 iMac M3 and its predecessor sits nearby, unused. As part of the installation advice Apple indicated what I should do to "clean" the old iMac and then send it to them for disposal.
I don't recall their advice.
Preparing an Old iMac for Apple disposal
