when you say prices are cheaper on this forum, do you mean i would buy them used from people?
The Amazon listings for S5s would also all be used, and Amazon is pretty strong on buyer protection (and a LOT LESS so on seller protection).
Their fees do tend to be high, so they tend to have some of the higher prices on stuff, it pays to shop around.
Also, keep in mind that the current difficulty flatness is probably a one-time abberation that won't repeat anytime soon, your income will mostly likely drop slowly 'till July, then the reward halfing will drop it in half (give or take difficulty adjustments - I DO expect to see a couple of difficulty DROPS shortly after the halfing though probably NOT enough to make up for the halfing in total, then start increasing again).
If i'm correct, the S5 is pretty stable, and since the diff rose and the halving is coming near, a lot of people get rid of perfectly fine S5's because they will not make any more profit with their current electricity rate.
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That's me, my S5s were more stable than my Internet connection, but at my current electric rate they were making all of $1-$2 a WEEK, so I sold them off while they were still worth a substantial fraction of what I bought them for.
Ditto the SP20, but it held it's value better.