I agree that the price of the yen has something to do with the current prices. But, I also think that the market is approaching capacity. We saw an incredible growth in the market for several years and this seems to have plateaued. And, in terms of DSLR sales, in decided decline since about 2012. So, I think another side of it is simple supply and demand, as in more supply of used gear and less demand.

To an extent, I think the supply/demand vs weak yen could be product specific. The obvious example is the 24-105 f/4 L. Used market used to be $800-900. But so many units were sold as part of a kit that the price now sits at ~$500-$600. But I think this applies to other lenses as well, likely the more popular. But, more niche lenses may be less effected.

I expect the supply/demand argument also applies to what we are seeing with bodies. I believe we are approaching a point where most that desire a DSLR has one, so demand has gone from "growth" to "replacement/upgrade/new entrants" cycles.