I'm ramping up to do it again now that inventories are shrinking in my area.

Where larger investors sell down to smaller investors and the asset pool shrinks in size to match the investor level.

I would think they would have vacancy issues as the tenant pool would shrink the higher the rents?

And that city services can't shrink as much as they need to.

The other main challenge is the window of time to do due diligence will shrink once you have it under contract so you'll need to have a buyer locked and ready to act.

We will convert to 3/2Estimated Rehab is $30kEstimated ARV: $195kRehab Details:Rehab started on 10/20 Compared to our other property this is much more thorough rehab :)Hired contractor to rearrange space, create 3rd bedroom, fix flow and update bathsRemoving old tile throughout (glad someone else is doing this, what a mess)Paint, flooring, new baths, light update of kitchen, etc6 week timeline but there are some things lining up to maybe shrink this to 5!

I'm as shocked as anyone that Philadelphia and some other cities are finally gaining population as the trend for decades has seen shrinking population in cities. 

If numbers shrink, we'll be on the other side supply/demand curve.

I recently read a book that talked about fortune building, and history has shown that emerging big players, along with more an d more competitors entering  a market, leads to  shrinking profit margins and impedes growth.

Actually, @Stephen Schaefer this is far from guaranteed:Population is only going up.In a number of countries, Russia and Japan, for example, population is shrinking.