When the PPQ first got popular it really exploded. Locally and online they sold like hotcakes. I think they sit on shelves now because most of the people that wanted one bought one. I don't see them coming up for trade up very often locally. In fact of striker fired pistols they probably come up the least. I think the retention rate for their sales must be good.
They are not common on the used market IMHO because their sales lag behind the other major player s in the poly market. Glock, XD and M&P sell a lot more new guns so there are a lot more used guns of those makes.
I sold my PPQ off it was reasonably priced PPQ M1 with 5 mags total for $500 it took forever to sell.
Around these parts the Walthers are selling well at a large volume LGS not that far away. Last spring they had a factory shoot with all the big vendors and Walther was there too. The Walther booth was about the busiest. It took me quite a while to try out a few of their pistols. The other vendors were pretty much no/low waiting.
This IMHO is not the case nationally. Everything I have read they are lagging behind and I know for a fact they are working with aftermarket custom trigger makers to help develop better competition triggers for the PPQ because they need to more more poly/metal. Internally they know it is not a big seller.
I am not saying they are bad guns they aren't but outside the world of gunboards and hobbyist they are not well known and not selling well to the general buying public.