Sorry Man.The key difference is that Neil's is unlocked and the new user won't be subject to a contract if they wish to go on a month to month plan.
Sorry I had to jump in here as I work in the industry and actually manage the contract with Apple, and other handset makers, for a major Canadian cell phone provider so I know the actual costs the carriers pay for the phones (and what/why Neil paid that original amount).
Contrary to everyone's belief these phones are EXPENSIVE and when you see the low contract prices it's because the carriers are subsidizing them. The only way we recoup the costs are via the longer contracts. Month to month (full price) pricing of handsets in both Canada and the US are approximately what the carriers pay the handset makers for them. Carriers DO NOT make money off of handset sales.
Good luck in your sale Neil. You may want to be flexible on pricing as it's pretty well known that the iPhone5 is coming soon and will further depreciate your iPhone4...