Geee, it's ridiculous if I had to tell "one of the auction site" instead of one word that everyone knows very well. Being politically correct is not my value, because it's irrational, unlogical and false.
Lemonade: you might mix used parts and new ones. Try to buy used 1150 platform with i5-2400 and Gigabyte Z68 motherboard, I will tell you it's possible for about 135 pounds. I can't tell you where you buy it - sort it out yourself, Stulid says I can't use this name, because it is "somehow" competitive, like nobody wouldn't buy here nothing else (?).
Memory, case, PSU and HDD buy here, in OcUK, from link given by Stulid, maybe except memory (2x2 GB for 39 pounds for SandyBridge).
It's also possible to buy used (no, it's not competitive info as you don't sell such old cards, and used from another private sellers; so Stulid you don't know if I tell about
Members market or not

) GTS250 for about 25 quids.
IMO 1150 platform is better, even with some "used:" injections than brand-new 5800K.