At a glance, that looks like a nice IBM 5150 machine.
Prices on these can vary quite a bit depending on condition, how early it was produced, and what is installed.
In this case, starting low on eBay is an OK idea, as a genuine IBM should get enough attention that it would get bid up to what it is worth.
If you found a local buyer, just a guess perhaps $150-$200 or a bit more depending on details such as how early the unit is. But remember that with a local buyer you save money, hassle, and risk by not having to ship or pay eBay fees.
You absolutely do not want to skimp on shipping, especially because those monitors are fragile. Professional packing/shipping is a good idea.
If you list it on eBay, I would recommend including a clear high resolution photo of the CPU unit's entire back. That helps show what is installed and gives a better idea of condition than the plastic front bezel. Even better, I would also recommend sliding the CPU unit's cover off and taking a top-down photo showing the motherboard, power supply, cards, and drives.
The fact that you seem to show it running is good.
(BTW, uploading images to the vcfed.org forum shrinks photos so they are 99k or less. Also some photo applications seem to use meta-data to indicate image orientation yet most software such as this forum does not support that.)