Understand that MacBook Pros made since about 2013 are not designed to be opened and examined. If someone has opened it, you don't want the computer. If you are concerned with getting a factory-fresh computer, you are shopping in the wrong marketplace.
As there are several issues that can come up with used Macs, including the very serious and common one of the seller not properly resetting it to factory settings, I general advise people NEVER to buy used unless they are fairly adept at Apple troubleshooting.
If you are a new user and do not have the skills to do serous troubleshooting, I recommend you purchase directly from Apple to avoid a financial disaster. You can often find refurbed Macs on the online Apple Store site at substantial discounts that are fully warranted and are, based on my family buying over a dozen Apple Store refurbs, superb values.
See this Apple online Store section:
http://www.apple.com/shop/browse/home/specialdeals/mac
The offerings can change hourly so do not be disappointed if the model you want is not listed at this time. Check back in a coupleof hours and you may find it.