To be honest as long as the seller has previous feedback of selling similar items and it is stated as being locked to o2, unopened and brand new and of course 12 months warranty from the purchase date (unlikely) you shouldn't really worry.
You can actually purchase 'iPhone' o2 SIM cards off eBay that have the free web and wifi bolt on for 12 months but the phones will work with any other o2 SIM card but you will have to pay for the data bolt on which currently is £10.99 (not to be confused with their £7.99 package)
If the seller has 0 feedback then dont buy it, if they have low/negative feedback about similar products dont buy it. Use your head. eBay isn't 'that' bad. I've so far purchased 3 iPhones from eBay and haven't had a problem with them. Also purchase upwards of 10 iPod Touchs from eBay with the same story. As long as the seller is respectable you shouldn't have a problem. You can always disput the item should it not be received as expected. E.G. Barred on UK networks, broken/damaged, not un-opened, not a UK iPhone etc. They are all grounds for misdiscription and can easily be disputed to allow for a full refund.
Good luck.