Please return the computer for a refund. The computer must be setup per Apple's directions in What to do before you sell, give away, or trade in your Mac. When you get a Mac that has NOT had these instructions executed you are left with an unusable computer!
I am guessing you do not want to invest in a new Mac, if that is the case you have options that are much safer. First, Apple offers Apple refurbished Macs. These are identical to new, come with the same 1 year warranty as a new Mac, are eligible for AppleCare and can save hundreds of $. To find these, navigate to apple.com, scroll to the bottom of the page and click Apple's Refurbished store.
Next, if this is still not a low enough price, go to a reputable reseller such as Macsales.com. Never ever buy a Mac from places like:
- E-Bay
- Marketplace
- Nextdoor
- Craigslist
- local PC shop etc.
Be smart, especially when you are not experienced with Macs.