I've been seeing the benifits to having a NAS lately and have started looking for one on ebay and locally, am I rolling the dice looking for used or should I just bit the bullet and buy a new NAS?

I've narrowed it down to Synology brand as they seem to have the most user friendly interface, or at least the best paid advertisers out there. If buying new I think either the DS220j or DS220+ would serve my purposes pretty well. Not really sure if the DS220+ is worth the extra $100 more dollars for it though.

Unless I find a fire sale on hard drives I think I would definitely buy those brand new in either case.

2 drives? why bother?

FWIW  I have 6 stand alone WD NAS drives and a QNap 451+ with four 8tb WD Red drives.

I'm short of space and will be buying four 12 tb drives this summer.

One thing I learned.  Figure out how much storage you'll need then double that.  Simple axiom...data grows to fill available storage space.

WD usb external drives are often on sale for excellent prices.  You can shuck the drives and install them in your NAS box.

Hard to know used... it's obviously a gamble. As to the general question yeah I enjoy my synology.

The drives are the real expense though.

The problem with any NAS is that the controller can fry, probably leaving your RAID data unrecoverable without spending a good sum of money.  I wouldn't chance a used NAS.  Having said that, my Synology's are pretty old, and I have no issues with them.

Understand that if you don't have data in three places, it's not safe.

As stated above, a 2-drive NAS is pretty useless.  You want RAID-5, which requires a minimum of 3 drives.  Get a 4-drive NAS.  Drives are cheap.  It's the chassis, PS, and controller that bring the cost up.

(In before the "Build your own NAS" crew.)

So I thought a raid could be made with 2 drives, is that not the case?

I have too many projects to take on another and learn how to troubleshoot a homemade NAS right now.

How come a 3 drive NAS is much better than a 2 drive NAS? Is it common for more than one drive to fail at once?

You can make a form of raid with two drives, but raid (redundant array of inexpensive disks) depends on distribution to achieve redundancy, thus reliability.

There's no difference other than immediacy in a two drive "mirror" array from just copying everything to a second drive from time to time.

raid 5 or 6 distributes the data across a series of drives, to allow for one (5 - "p") or two (6 - "p+q") drives to fail while still maintaining data integrity, allowing for a new drive to be swapped in to rebuild the array without data loss. The 5 or 6 permutations "stripe" the data across all drives in the array, increasing read and write speeds over single drives or simple mirror arrays.

RAID-0: All drives in the array (2, 3, 4, etc.) are striped together to make a single volume.  No fault tolerance.  If one drive fails, all data is lost.  The benefit is added read/write speed.
RAID-1: Two drives mirror each other.  If one fails the other one technically can be substituted.  No added speed.
RAID-5: Multiple drives (2 or more) make up the useable space. Another drive is not counted in the useable space, but allows data to be rebuilt if any single drive fails.  There is a performance hit (relative to RAID-0)
RAID-6: Same, but uses two 'parity' drives, thus fault tolerance extends to two simultaneous drive failures.  Performance hit, but the added fault tolerance is highly beneficial.

There are additional RAID levels like RAID-50, which combine the benefits of RAID-5 and RAID-0, but these require a lot of drives and in most cases offer little price/performance benefit to the average user.  RAID-2, RAID-3, and RAID-4 almost never used these days, for various reasons.

RAID-5 and RAID-6 performance increases with the number of drives in the array.  I sell some RAID-6 SAN/NAS arrays with 16 drives (scalable to 80 drives.)  Multi-user 4K/8K video editing houses use these.

For most users RAID-5 with four disks is the sweet spot.  Good balance of cost/performance.

You need at least 3 drives to do RAID5 (typo?).  
Some RAID controllers will allow RAID 6 on just 3 drives, but most require a minimum of 4 disks.

How in the world does RAID-6 work with three drives?

The + models are worth it if you are streaming media, otherwise you are better investing in a unit with more drives.

As others have said, you can buy used but you don't know much of the history and there is always the chance for failure.

While it doesn't happen often, we've actually killed a few DS1621+ series Synology's (under warranty) so even they can fail. Personally, I run a fairly basic NAS at home that uses mdadm (software raid, not hardware) on an old HP Proliant N54L Microserver I bought new nearly a decade ago now. In fact, I just ordered some larger "renewed" drives for it (upgrading 4 x 1 TB NAS drives to 3 x 4 TB Enterprise drives) as I've been heavily using space here lately (media creation/storage takes space). I'm just running Ubuntu 18.04 with SAMBA and it serves my needs perfectly (even some light Plex streaming) but I originally built that server in Summer 2012 with Ubuntu 12.04 (I think I skipped 14.04 and went to 16.04 then 18.04) but all of the more recent upgrades have really been to future proof the box. For example, I can take the drives out and put them in new hardware and most linux distros automatically recognize them as a mdadm array out of the box. I'm contemplating upgrading the motherboard to an ODROID H2+ and simply reusing the existing chassis...just keep on using it.

Holy data batman.
Did you download ALL the porn?

My co-woker has about 40 TB of occupied storage on 3 different Synology's right now. Let's just say, he really likes his movie/show collections and he has a lot of stuff (Dallas, CSI, Law & Order's and other shows that span a decade or more take up a lot of space, even at 480p).

It's funny how things change.
I've done CAD/CAM and IT for over 30 years. In the past we had the best of the best computers and data storage.
Now bitcoin farmers, gamers have better GPU and movie watchers have more data storage.
I remember paying $15K for a HP Unix workstation with 512k of memory and a 19" color CRT in 1999. That equals $24K today.
How do you backup 40TB, another 40TB ?

Pretty much.

wrong.

you back up 40TB with 2x80TB in 2 different locations

2 drives make up the usable space.
For example, 3 x 3TB drives will result in 6TB usable space.

It doesn't.
4 x 3TB RAID-6 results in 6TB usable space.

RAID calculator

6TB RAW, formatted with something like XFS, it's about 5.76 TB.

Build your own.  Older used tower off c-list or similar (pawn shops can be good, find a $150 desktop and offer to trade three boxes of 9mm for it....).  Before buying open case and look for leaky capacitors and make sure there are at least 4 sata connectors - if no leaks and enough connectors, boot with a Linux live CD/DVD (I like Mint for this) and make sure it boots and has at least 2gb ram (1gb will work, but don't pay more than $100).  Hit newegg or where ever for 4 old fashioned SATA drives that are the same size.  Boot and install Debian, set up software RAID-5 w/ a hot spare during install, install Samba and Owncloud, create some users and go to town.