Offered a cheap iPhone 3GS, but already have an iPhone 4 - Should I buy?

[danetello]

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?

 

[ucfgrad93]

Personally, I keep my old phone as a backup. I wouldn't spend money for a phone that is going to just sit in a drawer and not be used.