How and where is it better to buy a monitor 25-90 inches, especially 4k?

Question

Monitors

Dear Monsieur techies
Kindly tell me and share your experience.
I think to buy a huge second-hand monitor for the blues and for surfing
. Well, for games.
Interested in 4k well, and to a minimum of harm to the eyes.
Well, I want to buy for 5-10 years so that for 4-10 hours a day
What to pay attention to? avito or alik?
budget 10-15t.r.
Is it theoretically possible?
It was just 22" tft and now crt17. I want a normal window to the world,
but I worry that there will be enough money and not with horizontal stripes