I just bought an iMac from someone, but they’re still logged into their iCloud

I suggest that you provide the previous owner with the following support article so that they can properly prepare this Mac for sale. They should have done this before hand.

Ref: What to do before you sell, give away, or trade in your Mac - Apple Support

Optionally, you can do the last step (Step 7) in this article which is to completely erase the iMac's system drive and install a "clean" copy of macOS to bring it back to its original "out-of-the-box" state. However, if they enabled a firmware lock or used disk encryption, this may prove difficult. I second den.thed's suggestion to have the original owner take care of this ... or get your money back.

You can do it without contacting the owner but I suggest you do.

If the iMac does not have a Firmware password you can follow the steps here, you can check by turning off the iMac and then turning it on instantly holding ⌘ and R as soon as you see the Apple logo.

https://support.apple.com/en-us/HT208496

You need to return it to the original owner and have them properly prepare it for resale.

see > https://support.apple.com/en-us/HT201065