They were launched about 1.5 years ago, in summer of 2016. But AMD released their replacement (RX 580) in April of 2017, and with all the hype around cryptocurrency mining the remaining RX 480s disappeared from the stock almost immediately after RX 580 showed up. So now, 7 months after RX 580 release, I'm not aware of any store out there that have RX 480 cards in stock. It's just unlikely that the guy that's selling you these rigs managed to buy 6 new RX 480s just two months ago. Possible, of course, but not very likely.
 
I don't know whether you can build a better system for $6.5k, I'm only commenting on the price of these 3 rigs based on the GPUs included. It's all used hardware, and you didn't post any specs (other than the GPUs), so it's hard to calculate without knowing what it is you're buying. New 1070s are each $430 or less, new 1060/3 are ~$200 each, and new 580/4 are $210-230 each. So all the GPUs are close to $6k if bought new. You can run 20 cards with 2 rigs. 8*1070 = ~1200W, 6*1060/3 = ~ 700W, 6*580 = ~ 900W (ballpark values, actual consumption could be less), so you need ~ 3 kW of power in total between 2 rigs, that's something like 4 * 850-1000W PSUs = $600-1000.

Ok, I'm too bored calculating this stuff, and it doesn't really matter. The point is that even if you add the price of the remaining components (2 motherboards, 2 cpus, 2 memory sticks, 2 ssds, 20 risers) you're still probably under $8k. But that's all new hardware. Whether it's worth it to go buying used stuff for the kind of discount you're getting is up to you. If it's a friend you're buying this from then it's one thing, but if it's a random dude from craigslist.. then I'd rather go with new parts and build whatever and however I want instead of getting someone else's stuff.