buying in volume decreases the amount you spend on shipping, decreases unit cost, etc for both miners and PSU's.
Sometimes, when you CAN get a volume break, you can save a little on cost through bulk buys.
I doubt it would be make the difference between making RoI and not though, unless someone was right on the edge already.
I did say that amount invested has "pretty much", no "no" effect on profitability after all.
At most it's a very minor consideration compared to the other 3 - and is largely covered by my criteria 3 when it comes to the bulk purchace aspect anyway.
You can get small volume breaks too by picking up deals from local people wanting to hand off their miners easy, quick and cheap. I got a S4 for 425$ that way, a S5 and looking at a couple more right now.
The amount of good deals are very limited, so only suited to someone with time but only a small miner capacity.
yeah if you have elec costs on the low side you can make a killing by getting used equip ...in the midwest usa where I'm at the elec rate is fixed at around 13.5c kwh (xcel) this of course includes all fees/taxes. you only can get a better rate (and it an't much maybe 3c) if you use over 3.5kw a month (I have 2 titans plus house currently using 2.55kwh)
so i suspect their is a great movement EAST to Oregon and China etc of a lot of used scrypt and sha-256 equip
the circle of asic life....chip to doorstop with space heater as a side effect