Yes, it's pretty easy to buy them second hand.. 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.

The release date of the S5 was december 2014 (i just googled it, and used the first date i found, might need to be double-checked). You cannot buy new ones from bitmain, so if anybody sells you a new S5, it's because they had it in stock for months and months... You're probably better of buying a cheap second hand unit anyways...

IF you're going to buy a new one, consider an S7 or an avalon6, if you have a really good electricity rate, you might even break even in the long run with those machines.