i did i couple of times, but its a lot of trouble and theres not always a lot of profit in it. On average people will pay about 25% more of how much you paid for the parts. So on budget systems that can be as little as just £50 but obviously can get a bit higher on extreme PC's.
The problem with the people buying the systems on ebay is that they dont know a lot, so someone sells a real budget system at £500 and they are like "wow - thats cheap" and the actual good system at £1000 they completely miss out, not knowing that the extra money is actually gonna boost performance sooo much, so low end systems sell a lot better than high end ones IME.
dragon2309