2013 21.5" iMac Display Issues

Welcome to the Apple Support Communities.

Unfortunately, this is a hardware issue, and since your Mac is obsolete (Obtaining service for your Apple product after an expired warranty - Apple Support) it cannot be serviced. This means that, unfortunately, the Mac needs to be replaced.

Here is Apple's current Mac lineup: Mac - Apple.

Apple also sells refurbished Mac computers: Certified Refurbished Products - Apple.

Jack

+1 on Jack's assessment. Your 2013 is now 10 years old and should be replaced. You have lots of options available to you, most of them would be far superior to your current machine. If you need some suggestions on replacement options please feel free to ask and we can offer several for you.

My guess would be the GPU has failed, however only a professional assessment by an AASP could verify that. I would not recommend attempting to sell it, I would recommend using Apple's recycling program to get rid of it responsibly.