I would say that it is more based on reputation that actual sales.  The reputation then has a flow on effect to price and also availability.  Guys that think they have a title with a good reputation will typically be less likely to sell them which in turn makes them harder to find which in turn makes them more desirable when one finally does come up.  It turns into a vicious cycle. 

Take WH2O for example.  It wasn't that long ago that this machine was a definite B title and there were always machines popping up for sale.  Now their reputation is on the up and up as being an underrated machine, they come up less often and when they do, they are demanding higher prices.