I think you would be surprised at how many legitimate traders use eBay as a second "store front" as it were. A lot of eBay sellers have their own shops as well, so it's not just some shady virtual store run out of guy's bedroom.
Anyway as I was stating there is virtually no difference if I were to build it myself so I am gonna be buying the second pre-built rig (
http://www.ebay.co.uk/itm/370916574300? ... 1438.l2649) and adding my own PSU and a decent GPU at a later date. The seller in question has 33505 feedback so I think I am in safe hands