Does anyone have any other ideas to solve this? It is so old there is even a CD slot... curious if OS can be installed that way.
Yes. "Early 2011" iMacs shipped with operating system installation media (DVDs) that need to accompany those Macs from cradle to grave and should be included with the sale. Apple stopped including those two grey discs beginning with the "Mid 2011" iMacs, because they incorporated the Internet version of OS X Recovery. Certain earlier Macs could also use OS X Recovery, assuming their firmware had been updated: Computers that can be upgraded to use OS X Internet Recovery - Apple Support
Assuming you don't have those discs, your next course of action is to purchase a retail, white, Mac OS X Snow Leopard DVD. Apple no longer sells them but you may be able to find one on the used market. Boot from that disc, erase the Mac, install that operating system, and update it using Software Update. That will result in OS X 10.6.8, which will incorporate the App Store, which conveys the ability to upgrade to the latest macOS version compatible with that Mac (probably High Sierra). Read How to download macOS - Apple Support. Scroll down to High Sierra 10.13. Follow its instructions (use Safari).
The best option is to just sell the Mac in its present condition. The lack of an operating system should not deter a knowledgeable buyer of used Macs — and you should not sell it to anyone other than a knowledgeable buyer. It would just result in frustration all around. You would be far better off "recycling" it.
