I cannot find any
written policy online that states that unlocking the Nexus 7 will void the warranty.
I checked the following locations:
"
Nexus 7 limited warranty" on Google Support site
"
Support Nexus 7 - Warranty Policy" on Asus site
With that said, I also don't know where I put the paperwork that came with my N7, so I can't say for sure that it wasn't mentioned in there somewhere.
As a general best practice, think of it as you would most other manufacturer warranties. For example, most firearm manufacturers say that their warranty is void if you modify their weapon with aftermarket parts. But 99% of the time that just means, "We won't support this item if you were the one who destroyed it." For example, if you asked Glock to look at your brand new pistol because it was keyholing, I doubt they'd say, "Well, you put in a TD magazine release. We can't support it."
If you unlock your Nexus 7 and later have hardware failures of some kind, I would bet you money that it will still be supported (assuming you're still within the warranty period). If you unlock your Nexus 7 and then screw up the entire system and file structure, don't expect Google or Asus to fix it. That said, however, it's absurdly easy to start the device over from scratch, and the device can be "re-locked" back to a factory state before you send it in. If it's in such bad shape that you can't even power it on enough to reset it to factory status, then it's in bad enough shape that they won't be able to tell what you have or haven't done with it.
I've never had experience with Google/Asus support directly, but I had to send in a Samsung Galaxy Nexus for warranty replacement because the charging port was damaged. Before I sent it back, I reset it back to stock and locked it up again (I had previously unlocked it and customized every facet of the device). The effort wasn't even neccessary though: they sent me the replacement before I sent them back the defective unit, and it was obvious it just went to some factory for destruction/recycling/refurbishing/whotheheckknows.
I'm not sure what to say. Personally, I would prefer to purchase directly from Google.
If the issue is saving money, I would check to see what promos are running. For example, Google sometimes gives Play Store credit with device purchases. You could use said credit for music, movies, apps, or games.
If the issue is having the device unlocked for you, I wouldn't buy it that way from someone.
The process to unlock the device is very simple. The longest part of the process is waiting for your desktop OS to install the drivers necessary to communicate with your N7 over USB debugging. If you aren't yet ready to undertake that process on your own, then you won't be comfortable doing the stuff that actually requires it to be unlocked. Flashing a new kernel, for example, is a more complicated process than unlocking the device to begin with.