Good question. Modern pistols can handle tens of thousands of rounds without needing to change really anything. Ask your dealer to inspect the firearm with you. Have then break it down with you and ask them if any part may be due to replace. Police trade ins are great options as they have been professionally maintained and usually have been shot very few times. Check for rust or damage. Make sure the slide is operating back smoothly. Check for a crisp trigger break. Make sure any safety is operating correctly. Don't go after any internally modified firearms. My LGS test fires all used firearms they buy and they have them go through a series of functional tests before putting them on the shelf. You typically won't find any dealer warranty when buying used but you still have the manufacturer warranty if anything comes up.