May I ask if locally is Kazakhstan? If so, I would imagine the market for second hand Mini's might not be so strong..
I had previously followed your posts with interest regarding the Mini. I too purchased an i7 mini with egpu, became disappointed with the performance, and have demoted it for a custom built pc (best computing decision I have made for a long time).
Did you become too disappointed in yours to carry on using it?
I suspected the resale price for a second hand mini would not be worth selling it so soon, so it has become a family computer.
Mac's are really easy to sell. If the price is right. If no one is responding. Then you are asking way too much. That's my experience with sites like Craigslist. If you ask too much you won't get a single call. Once you hit close to the local market value. You'll get flooded with calls for any Apple device.
I would have no clue as to what local used pricing should be in Kazakhstan or the best websites or apps to sell in your location. Looking at US pricing for a used Mini. They sell for 20% to 30% less than new. Depending on whether or not they still have AppleCare and original packaging.