That’s usually due to a faulty analog board or flyback transformer. This repair really needs to be handled by someone with the appropriate experience due to the voltage of the CRT and the fact most of these have burned out bleeder resistors. These can hold up to 15-25k volts, which KILLS. Take this to someone who is able to work on a CRT to see if they are willing to take on the job.
More then likely you’re better off getting a G4 Mini to replace this due to the cost likely being about as much as the computer itself. The problem with buying a used eMac is the price of shipping (local only) and a lot of them have CRT screen burn (e is for Education). School districts loved to leave these CRT based Macs at the login or another situation where they gauaranteed the machines would have burn-in due to negligence by the time they retired them for Intel iMacs - granted some didn’t get burned due to IT departments putting screen savers on, but most of these are still affected.
The eMac is not a collectible - but the OS9 boot capable ones are mildly valuable since most are not able to do so. Part of that is they're too common and most of them are EDU spec, which is generally not a desirable trait. Combine that with high shipping cost and you can't really sell them for much and they're a local only purchase. A Mac Mini is not.
Unless you need OS9 support, that’s probably going to be your best bet since PPC Macs are primarily machines people keep around to run legacy applications that are PPC/UB (and in some cases, hardware like older printers with a basic driver in current versions of MacOS, but the utility only works on their iMac G5 ALS for example) or they use something like FCP 1, which REQUIRES an OS9 machine. Get a system that can boot OS9 if you need it and that's why you have this eMac - the Mini does not support OS9 boot but it supports emulation.