I'm also getting ready to sell my 2017 21.5" model, although mine is the 4K with only 8GB RAM and regular HDD. You don't say where you are, but in the DC area in the US I see a lot of these advertised in the $200 range. I suggest starting a little high and lowering the price if it doesn't sell. I would mention the 4K display - a lot of the ones for sale here are the base model without 4K - and if it has an SSD instead of fusion, mention that. Say that it's had all its updates.
Otherwise, I plan on taking nice pictures, including the original keyboard and mouse, screenshot of the about this mac window. I'm pretty sure you can get more than $90-110 for it.