Iphone 4S Users: Most Common Functionality Fail With Iphones?

I'm planning on buying a used Iphone 4S and read that the the most common failures on Iphones are the home button and the power / lock button.

Are there any other parts of the phone that I should check for damage? Or to see if they're working or not working (physical or any programs within, etc)? Or is the home button and power lock button pretty much the only thing worth checking?

Check for water damage in the usb and headphone jack

Also check speaker

Power button and home are main issues

When buying the iPhone 4s used. Ensure that the iPhone has a clean install of iOS.

Also check for the basic functionality of the phone.

1. Check whether the headphone jack works or not.
2. Check external camera and speakers
3. Check for damage
4. Also ensure that the phone does not have cydia if you don't know about jailbroken iPhones.
5. Do compare the prices of used iPhone 4s and refurbished iPhone 4s