I do not use my phone much but I would like to have an iPhone. I plan on buying a used 3GS or alternatively an iPhone 4 if I get a good price. What should I look for before I buy? I do not want to end up with a stolen phone. I plan on making sure the seller has the original box and all original accessories. Anything else I should ask for? Physically, is there any weak point (like the Home button) I should check?