Since you have not listed every item brand, model number, age and condition it is very hard to figure what you have. However a general rule I figure to resell a brand new system just bought say with in 30 days, it would be worth no more than 75% of what was paid for it. I would never pay more than 40%-60% of the original cost of the parts for used. If it was branded (ie. HP, Dell, etc.) maybe even less as most box units are old before you get them. Systems from builders like Origin or I Buy Power have more value.