Some questions for you:
When you go to sell a property you own, are you going to sell at the market price or are you going to sell at a discount to help out lower to middle class families?
Everyone is going to take whatever they can get because things are so expensive including replacement housing you are almost forced to do it. Had we regulated the transfer of single family homes in some way, things might not be so bad.
Sadly, most average people cannot afford to sell "at a discount" to what some fat cat will pay to add to his large portfolio because of the housing mess we are in.
Are there any major markets in the US where a single person or group has the market power to charge what they want because they've captured enough of the market (like a Pottersville)?
I am not sure, but there is the market price. If you have enough places charging an average price for something (like one bedroom, two bedroom, three bedroom, etc), most investors will charge that price or maybe higher if its in a better location.
Kind of like gas prices. A long time ago, it was easier to find lower prices if you drove around enough. these days with prices posted in central places, most stations charge that price
And if there are any cheap, good places to rent they are gone very fast.