He may not be far off the mark there. What you need to realize, possibly, is that you basically got very lucky on your other unit, to get $300 for that. Basically you found a newbie. Once he gets the unit home, and if he tells the truth about what he paid, his buds will tell him he coulda bought that unit right in his home town for a lot less, lol. Then he paid shipping on top of that.
Im not being sarcastic, just honest.
Then again, youre talking to someone, me that is, who can build any unit you want, so why would i buy yours. Its harder for people like me to put a price on these units.
However, with new or fairly new pcs like the one you are talking about, its actually pretty easy as these new pcs sell for the sum of the parts. Be honest about the value of the case, the mobo, the cpu, ram, hard drives, keyboard, mouse, vid card, etc, etc. Add them all up, and if you put a fairly honest, lower end estimate of the value of these items, even the os if you are offering a os with a cd and honest password (not warez) add them up and there you go, thats the value of the pc.
This may not work quite as well with older ones like your last one, the P4 1.9, but still you could do that. In your case you found a buyer who is willing to pay more for a unit ready to go. Maybe he lives on a farm way out of town, something like that. For these people its important to have one ready to roll when plugged in. And also, it should be obvious, hes a newbie who cant build his own, lol.
I have to kinda laugh. I'll tell you why. Ebay is the worst place to buy something. I can get you anything, a cpu, a whole pc, high end vid card, etc, etc, for a heck of a lot less than you can buy it at Ebay. There are so many good forums with their own "for sale" section. To give you an example, i have bought probably 500 to 600 pc parts thru these forums and NEVER have i paid Ebay prices. We call Ebay Fleabay, its a joke. Its too bad they gave up the "for sale" section here without even giving it a proper try. I am probably gonna get banned for saying this, but i dont care. I like this forum but now that they have killed the "for sale" section i feel a lot differently.
If anyone wants to know what good forums have good "for sale" sections just shoot me an email and i will tell you. You can get far better deals there than that lousy Ebay. Of course, if you want to get top dollar for your own pc, then Ebay is good for that, lol. Sometimes, like you did, you find someone who will overpay.
Hope you dont mind my honesty.