Since I don't know the apple of the used iMac that I purchased for my parents, can I use the apple id that I have for my iMac?
appleID is the social security number in the world of apple
you should use it for all devices you ever use and you should never ever ever share it between users
What you need to do first is to follow the steps in What to do before you sell, give away, or trade in your Mac - Apple Support and then you can set it up. NEVER EVER EVER accept a used Mac until the items noted above have been done. If I understand correctly you want to have your parents use the iMac, if so they should have their own Apple ID and not use yours however you could do that. Not advised, but you could do it.
Can I use my apple ID for a used iMac that I recently purchased?
