I have been offered a cheap iPhone 3GS 8GB for £100 (around $170). It is fairly used and has got a few scratches but the seller says it is still in full working order. I think I could use it as a back up for my iphone 4, or I could sell it. I know I could definitely sell it for more as a similar phone has sold on ebay recently for £170. So is it worth getting a backup iphone?