Okay.Umm
Because under warranty, used i5 10400 is selling for 10k
To be blunt: It's old, the i5-7500 is five generations old. It doesn't have hyper threading. There's no warranty left. You can't buy new motherboards for it outside of the stripped down H110.
Since AMD's 1st gen Ryzen came out in 2017, CPU's have dropped in resale value considerably, perhaps for the first time since 1999-2000 where we went from 550MHz to 1GHz in less than a year. The Ryzen 7 1700 was being sold for 10k new before it discontinued, that's 8 cores and 16 threads. The second hand market is saturated with high core count/thread count processors that were release in the last few years. And that's not even counting the decommissioned Xeons, you can get 8 cores and 16 threads for under 4k if you have a compatible motherboard.
A 10th generation i3-10100F is 7500 new, and the i3-10100 is 11500 new. Both of those processors are 4 cores 8 threaads and are 15% faster in single threaded performance and 45% faster in multithreaded performance.
You may be able to get upto 6500 for it from someone who doesn't know any better.
Thank you
For explaining