I find Ryzen 5 1400 really rare and there are so few people that own it. Is there a reason behind it?
I was thinking of buying it to avoid too much bottleneck with my gtx 1070 as my current i5 2500k just bottlenecks it too much. But for some reason it is impossible to find a 2nd hand one. Would like some answers about this rare processor.

Pls dont ask why I use old hardwares, it's obvious. I'm dirt poor but gaming must continue.

Your 2500K system still works right? So why the rush?
Save more money for Zen 2 (Ryzen 3K series). Even better if you go straight to Zen 3 (Ryzen 5K series)