Some of those spikes were very short lived. I've seen in recent weeks, for instance, the A50 f1:1.7 go for anywhere from $45 to $90. Just like any other day, it depends on who is looking, and what they are willing to pay. List your stuff at what you want for it and see if anyone takes it. If not, relist. If Still not, you may have to lower your price. If you're buying lenses to resell, you have to figure on getting bottom dollar on them. Otherwise, you paid too much if resale was your goal.
That said, the K5 does quite nicely with MF lenses. I use it where it's available to me but I've never thought AF was all that it's cracked up to be.