£350 is too low for a gaming PC build. You should just get a PS4 instead to be honest. If you can squeeze out £500 you can get a decent one, and nobody can make a £50 profit of a PC if its been under a desk for a year. New hardware comes out every year and the hardware in the computer is outdated.
So no, you won't be able to sell it for £400 as you planned. I'd recommend to do some ACTUAL research.
Hi sorry but I have £450 to upgrade my PC. I will keep my original Optical Drive,Hard Drive,OS and RAM could you suggest a upgrade for the CPU, GPU ,Case and Motherboard and maybe a liquid cooler Thanks ( I want to play games like FIFA 14,COD BlackOps 2 and BF4 )