My rule of thumb is never spend what u can't afford to lose! Having said that EBay protect their buyers much more than their sellers, so provided you make sure u buy on ebay UK and pay with Papal ( & check out the seller as suggested above ) you should be ok!
There has been a huge rise in second hand mobile phones recently, you see all the adverts for people like Mazuma and others. Some fo these do end up on ebay as mobile phones are more like a currency now and sold by the tonne!
You can always compare new mobile prices at <a href="click here" title="Compare Mobile Phone Prices">click here;/a> and see if others apart from CPW have a better deal.