Depends on how much, and the condition of it.

Is there any burn-in, or image issues such as with image geometry or the colors?

Is it lacking contrast? If so, the tube's probably worn out or the contrast is set low.

I'd personally go for it, but in the end it's your decision

It's 5 EUR 😁

This monitor would have been ok for most people in the late 90s and makes for an authentic retro experience. It's not one of the better CRTs though.
Keep in mind that CRTs flicker so in order not to be annoyed and fatigued by it or get a headache you want them to have a high refresh rate at all resolutions you're going to use.
Some people will be ok with 75Hz, almost everyone will be fine with 85Hz. This monitor supports 1024x768@85Hz but only 1280x1024@67 which is likely going to be too low.
So from an ergonomics standpoint you will be fine unless you want higher resolutions than 1024x768.

I have my modern monitors for higher resolutions. This one will never go over 1024x768

Ok, I have it now!

Certainly not the best monitor I've ever owned, but its tube is still in good condition, works perfectly well in lower resolutions, better than modern ones trying and failing to downscale properly!

1024x768 at 85 Hz:

Congrats! 1024x768 is as high as I ever go for retro PC gaming, looks like you got a nice deal for €5.