Now, id' the device made it much easier.

Link for XDA - it has 10X the info you ever needed to know, running MM 6.0.x now, factory stock - most likely, previous owner restored to full stock, relocked the bootloader after unrooting, that's what these devices are for.
http://forum.xda-developers.com/nexus-7-2013

I have a 1st. generation Nexus 7 (2012) and travel with it, manually upgraded to MM 6.0.1 with a custom rom. You do NOT have to any of that - odds are good that the seller restored it to stock condition before putting it on eBay.

Look to see if it has a custom recovery - TWRP typically used or stock by Google. With TWRP, you can flash a stock image in *.zip format easily with a full wipe, etc. - read those steps & follow instructions closely - grab a cup of coffee while PC is installing drivers & software, before rushing off to do it. Otherwise, with factory recovery - you will need to flash the ROM image differently - best way is to use Android SDK tool. Otherwise, Nexus Root Toolkit in XDA forum is the easiest way to flash. This is a weekend project, not a rush one - to avoid turning it into a paperweight, spend the morning reading & reflash the image in the afternoon, reboot & setup - by evening, you will have a good-as-new N7 in your hands.

There is quite a bit to learn & get familiar with, which I won't go into here on a travel subforum. Google has a dedicated N7 forum for users to share & get help, along with XDA. Turn on Developer's Option & enable Android debugging mode - to allow full access to internal SD drive, and to check that Android Device Manager is installed & configured to work, that will give you a great deal of control with permission settings while using it, or in standby mode. Have fun