Many apartments never get put back on the market again as long time residents, pass on or move on. I know of one building in my neighborhood, that the remaining long term (co-op board) rent out the vacant units as if they owned the whole building and are profiting from it! Sometimes it is very much a matter of, you need to know someone that knows someone to get into them.
The better managed buildings are listed just like other co-ops on the open market, of course the buyer does need to be in the required income range and family size for the size of apartment, but the apartments can be sold to the highest bidder for what the market will bare.
They are generally priced lower than the average for the neighborhood that they are located in. They do exist in almost all neighborhoods.
It takes a bit of work to find them, but for long term stable affordable housing it is worth the effort.
Kathryn Lilly, Broker
Realty on the Greene, LLC