I wouldn't buy second hand parts for my main rig from anyone I didn't know and trust well. The parts may appear to be working fine and even look fine, but until you get to stress the PC over a longer period of time you will never know how well the parts are working and you will also never know the kind of wear and tear these parts have been put through before they ended up in your hands .
You can take the risk and spend all your dollars on it, but don't be surprised if they aren't working too well under stress or maybe some months down the line one of the components shits itself and dies. You could say I'm being pessimistic, of course these parts could be in great condition and work great for some years to come. But you will never truly know that and if they die on you, you have no warranty or anything of that kind to replace the parts you bought, leaving you with a non functioning PC and making you spend more money to replace the part.
IMO, you should be patient, save your $400 and keep saving that money until you can buy your own brand new rig somewhere inbetween low and high end, you can get some decent rigs for good prices that will run most brand new games on good enough settings to still look nice and run smoothly. The mistake some people make when it comes to PC's is that they aren't patient enough, they would rather blow their money on a rig to play stuff ASAP, when 6 months down the line they could have saved more money and bought a more far more powerful rig that will last a lot longer.
