Common issue with iPhone 3G/3GS are: Glass on the iPhone 3G/3GS is fused to the touch sensor, meaning they will need replaces at the same time. There is a rubber seal surrounding the glass that may deteriorate over time and as such may fall out upon removal of the glass. But can be replace for low cost.
Common iPhone 3G/3GS Repairs
iPhone 3GS Headphone Jack Repair
iPhone 3GS Exterior Buttons Repair
iPhone 3GS Charge Port
iPhone 3GS Battery Replacement
iPhone 3GS Camera Repairs
iPhone 3GS SIM Tray Replacement
iPhone 3GS Screen Repairs
iPhone 3GS Unlock Service