Get the proccesor and mother board new.Its the heart of a gaming system!
Yes you may buy the graphics used but dont buy the proccesor and motherboard used.
Rather get this config :-
AMD A6 LLIANO QUAD CORE 3650 around 6k (5k for tri core)
Gigabyte GA A55M SV2 (around 3.5k)
then get the other components used (if not going into high gaming,the A6 has an HD6530D in it.)