Just a few things that readily come to mind...
1) When inspecting a used MBP in person, make sure that the "About This Mac" is accurate. There have been several cases of online sellers changing the stats of the aforementioned, in order to defraud buyers.
2) Check the serial number online for warranty status etc. You want to make sure it all matches.
3) Make sure FileVault 2 has not been enabled and other iCloud services are logged out etc.
* Ideally, the OS should be a clean install waiting for it to be registered with the new user and iCloud account.
4) Another big issue is underlying damage (from spills etc.) that can't readily be seen without taking the bottom plate off. That is something you will need to contend with. It is a risk. Unfortunately, not everyone is going to be honest with you about what they are trying to sell.
In my opinion, it would be better to buy from the Apple refurb store, or off the forum here from an established member that has good standing.