Buying something used has a lot of risk. I would suggest you "Hard Reset" the device so that you can clear out any junk installed prior to your obtaining the device. This will then return it to its original state (before anyone used it).
If that doesn't resolve the problem, then you may want to consider returning it and getting your money back.
Additionally, how much space it says it has when new is never the amount you get access to. That amount is prior to anything, including the OS being installed.
"Hard Reset"....
NOTE: Doing this will clear 'all' the data on your phone, so it is suggested you not choose this option unless you have to, or aren't worried about losing anything on the device.
Be sure you back up any data you want...