I think Craigslist or Facebook Marketplace for cash would be better for selling your iMac. It'll avoid shipping costs and any shipping damage risk. Make sure to fully wipe the drive though. Apple products move fast. You could sell your keyboard and mouse on eBay though. Search by the A# underneath to see what they run for to see if it's worth it. The computer likely wouldn't be worth more than $80 USD though if I'm guessing, but double check on eBay (noting that shipping is often built in).

I sell a decent mix between eBay and Craigslist, but have recently added Facebook Marketplace. I've also sold some stuff on Etsy before, but not too much.

Since you mentioned, I recycled the top component of a Passap E6000 huge knitting machine I haven't listed yet without the power supply. I couldn't find the power supply, so it's an apartment decoration for now. I guess I'll be listing it untested soon for cash & carry. I sold the Victor Knitting Motor Counter that was on it for $75 though to get something from it already.

I also recently sold a vintage Bernina sewing machine I found outside (they're in high demand for some reason). That would've been perfect for you in hindsight. :(