Re: 21 inch, high res
I doubt it's valuable. I sent 3 off 19" 4:3 CRTs that were 1600 x 1200 and could do 1920x1080 with reduced scan height for correct 16:9 aspect.
No-one wanted them.
We have some lovely Benq QHD screens with VGA, DVI & HDMI. They work well at any game resolution on a VGA 3D card. Under €200.
Currently I use a 23.5" UHD with DP and HDMI. Under €300.
The HD and 4K TV sets with HDMI work well with cheap adaptors off Amazon and eBay.
All HDMI out:
1) Composite in. NTSC or PAL
2) Component in needing progressive. Some DVD players.
3) Component and Y/C in that works with S-VHS Y/C or DVD Component.
4) VGA in and HD HDMI with a 3.5mm jack audio in for sound on HDMI.
CGA can easily be adapted to composite or Y/C as it's basically an RGB version of NTSC using an old console RGB to composite adaptor.
EGA can be adapted to a VGA connector. So actually can CGA, MDA and Hercules, but few VGA inputs will take those signals.
CGA, MDA, Hercules or EGA can be adapted to component. A decent component to HDMI can take interlace or progressive.
Some HD & 4K TVs that have no analogue inputs still have an analogue tuner for RF, though those are usually PAL or NTSC, never both unlike composite & Y/C inputs.
It's only in last few years that the picture quality on decent LCDs has surpassed the top of range CRTs. My Samsung & LG 4K screens are as relaxing as eink to read. flicker free and can be calibrated for photoediting.