1st of all whats a Gigabyte GTX 285x2
(i take you mean 2 x gtx 285)
2nd of all selling full high end systems on ebay or anywhere else is going to be difficult, people spending that sort of money want parts they chose, not something someone else chose for them, thats one reason why companies sell systems cheaper than buying in parts.
3rd its worth going down to some of these computer shops and seeing if you can get a trade account
(can be fairly easy even if your peeps have a non related business), you'll get maybe 5% - 10% discount
4th 14 day warranty is not going to cut it, if someone is going to spend over £1500 on a new system they would want at least 1 year warranty (which you should be able to pass on as you will still have the warranty on the parts)
5th you mentioned dell, now and again they have top spec i7 systems for approx £600-£800 with a nice 20-22" screen, even adding a top end graphics card or 2 will save the customer a good £500+ over your spec
6th how have you calculated the paypal / ebay fees?
i think £1800 of funds incurs £61.40 of paypal fees
not sure about the ebay fees
7th if someone wants to spend a big wad on a pc I don't think they would want to restrict to the pc parts your offering, e.g. they might not like the case, prefer a different psu or graphics card, or might want vista 64
8th unless things have changed at scan, I wouldn't hold hopes on receiving everything within a week, there can be delays in receiving parts which will annoy your customers
9th about pnp, make sure that you take some insurance but then again insuring to £1500 can cost a lot
10th sending pre built systems can be a problem, a simple sata cable might just pop out in transit and the other end probably wont be too happy with you asking them to open up, also ive had a system sent to me with the cpu heatsink still in place, but during transit it dislodged and destroyed the whole system in the process
11... well im going now but im sure others can fill me in