If the PS Audio Stellar 700M is as great as all the reviewers say....

why are there so many used ones for sale?

Because the market is where the rubber meets the road. Whole lot of stuff sounds just great until you get it home. That is why a big part of learning to read reviews is learning to read actual user comments. All the gear I have bought for something like a dozen years now has been based entirely on reading on-line. Almost never do I ask any questions like on here. It is pointless. All the questions have been asked and answered a million times over. Search around you will find your answer.

Even this one right now. The answer is when there are a lot for sale then they really aren't all that, never were, probably never will be. There's way more guys looking to make a deal than build a system. They talk otherwise but there's two ways to judge- by talk, and by actions. By their actions they demonstrate their unseriousness.

So one of the last clues after doing all your research is to look at the used market. When you come up empty you know you are on the right track. When you have to wait for delivery, even better! These things are telling you people by their actions have voted with their dollars that these are the truly good components.

This I can tell you these new ps audio monos do sound very respectable 
adding a12au7 tube per mono block ,but what’s missing  is the realism  depth and liquid realism , for example I heard them vs the pass labs 60 class a integrated , ,the pass labs doesnot have the power or the slam ,but is far more natural sounding much more on the warm side triode like ,The other the latest Coda CSib integrated amplifier with the 250-500 wpc  12 first watts in pure class A , which is very tubelike in many ways more pentode then the warmer triode usung jfet,Mosfets, and bipolar  transistors on the outputs. 
but the leading edge is more defined having a separate class A preamp section 
Was very surprising and the best balanced of the 3 ,from  what I heard or to my taste. Based on that I ordered the Coda ,and just came in after a 6 week wait.,I  wanted the lower power  one which has class A first watts  of 18 then              150-300wpc . 

@n1dr folks are upgrading to the newer and more powerful M1200s. 

I’ve had a Stellar 300.  Very transparent, great tight bass. Needed 100hrs burn in, sounded bad right out of box. For me it was a little edgy with my Cornwalls, but Klipsch can get edgy quickly without a very smooth amp. 700 is supposed to be a little “warmer”.  As far as there being a lot of anything on used market, I think you have to factor in scale of new sales. A large scale maker, like PS Audio sells a lot so they is likely to be more for sale used.  Also, upgrading is a real addiction in this hobby, not really an indication of poor performance, we just alway want something a little better! 

M700 is their least expensive mono blocks and they do sound good, but their bigger brothers like M1200 and BHK 300 are in a different league. People likes PS Audio products often do upgrades and they have generous upgrade program. Therefore many trade-in such as M700 ended up in the used market.