You can use google & search from ebay if you wish to buy used items. However, I highly doubt if will sell a good i5-2500K for £80.
But in any case, an i3 is good processor (what is the number by the way? i3-2100?). You may be able to put that £80 to better use by buying a graphics card instead specially if you are just using integrated graphics card. So, what is your current graphics card?