Used:
Well mostly you can opt out for Haswell generation;
CPU- i5-4570,i5-4460 ,i5-4590 ,i5-4670 , i5-4690 , these are higher clocked ones with higher turbo boost which helps a lot in games.
Motherboard- Pick any cheap motherboard, i have 50$ one and it works perfectly.
RAM- Well if you are going for 8GB or 16GB DDR3 1600MHz is max because of CPU and motherboard limitation if you are going to buy cheap.
GPU- Now if you expect to run latest games without lag or 1050Ti/1060 6GB If you cant find those at resonable price then GTX 770 (4GB version),
https://www.youtube.com/watch?v=MkJ17pojMRw GTX 770
R9 280X, same as R9 390 R9 290.
As for Nvidia part GTX 970/980.
PSU-Now depends what you find for your GPU at resonable price, Seasonic G 550W series if you go for Nvidia GTX 770 and 970,
Others are a bit power hungry, EVGA supernova G2 650W would do its job.