Did you buy your used A2 for the same or more than the guy who sold it to you bought it new? The point is not that selling hardware secondhand is in general a bad thing - by no means am I saying that.
The point is selling hardware secondhand for more than it is likely actually worth in order to recoup your investment is a bad thing - which is to say, "making about 1/2 or so back on your money and you sell them used for just about the price you paid for them new" (if not more).
Unless something atypical like flat diff is happening (which can't be counted on enough to "rinse and repeat") and the viability of the machine is actually retained during that time, selling it for near new price after several months means ripping someone off. You get your "ROI" by effectively stealing it from someone else. That's exactly what's happening with Bitmain selling used S5 for the same or more than they sold them new several months ago. If the diff had gone up more than about 10% in the last six months (like the 50-100% that would have been expected had the trends of the previous two years kept steady), nobody in their right mind would be buying S5 for the same money they paid at Christmas.
I feel the argument should be well-qualified enough to require no further defense, and as it's pretty off-topic at this point, I'll say nothing further on the subject. How 'bout them BM1385 chips, eh?
Huh? I use the same strategy with my ASIC purchases and obviously there is a lot of guesswork and a little bit of luck involved, but I have profited off the S1, S3, S5, and SP20 by using this same strategy. When I decide to sell my old gear, I do so based on what I paid for it, how quickly difficulty is rising, what I have mined so far, the season of the year (save on heating bill in winter), and the approximate going rate for the miner on the second hand market. I list the miner on ebay at 0.01 cents with no reserve, offer free shipping, set it for a week, and thats it. Knowingly misrepresenting a miner as new, something it isn't, higher performance or better condition than it is, etc are all bad things. But if the free market is overvaluing mining gear at a particular moment in time, I'm stealing from someone if I sell it? That doesn't make sense. Some people have cheap or free electricity, some people dont care about ROI and it's just a toy for them, some people are simply making a poorly timed purchase because they don't know how to properly assess the value of the mining gear. It doesn't matter. As long as they are freely choosing to purchase the item and I am not misleading them, there is nothing wrong with it. It's value is exactly what the free market will pay for it. Sometimes the free market overvalues a product or service, sometimes the free market undervalues a product or service. If you are able to accurately gauge the true value of an item better than the rest of the market, whether it is the ASIC market, stock market, or hell, the grocery market, you will gain anytime you are able to sell something while it is overvalued and buy something when it is undervalued. This is just how a free market capitalistic economy and natural price discovery functions.