The signs that your monitor is dying are flickering, black lines, black screens, slowness to brighten, stuck pixels, missing half screens, purple spots, green spots etc.
Other signs of a monitor dying are inaccurate colors, dead pixels and distorted or fuzzy images.
However if you do have some flickering or green color on the screen it could be a loose cable that needs to be tightened or pushed back in fully or you may have a bad video cable to the monitor.
Buying used monitors is OK as long as they are working and as long as you can buy the monitor cheap enough.
I've bought used monitors and refurbished monitors and they've worked just fine for me.
Refurbished for monitors means that the monitor was either used but then restored and cleaned up or it could also mean that the monitor was returned but repaired and then sold again.
Refurbished monitors are worth buying if you can get them cheap enough.
I've bought refurbished monitors for $30.00 a piece and they've worked fine for 4 years now without any issues.
If I needed to pay more than $30.00 or more for a refurbished monitor then I would just buy a new computer monitor.
Refurbished desktop computers are worth buying as long as you buy them from a reputable seller.
I've bought refurbished desktop computers on eBay and have never had any issues.
If you're just needing a cheap computer for causal browsing, work from home etc then a good refurbished desktop computer can be good enough for most people.
I work from home and use a good refurbished desktop computer that I bought for around $100.00 from eBay and it has served me well for 2 years now.
I would buy another one if needed.
I don't do any gaming or resource intensive activities on the computer so a good refurbished desktop works for me.
If you cannot afford a new computer then a good refurbished computer will be just fine.