As far as I know, the Private Market only shrinks when turns arrive when I "login". That is when the game gives me turns it decays it.

I destroyed 1 building and my PM shrunk and then I dropped that acre and my PM shrunk as well.

Is this suppose to happen?

not sure about the building, but when you dropped the acre your pm is land based.. somebody could tell you far better than me, i think. and yes, turns while logged in decrease your pm.. not sure by how much, again somebody else ;)

That is the intended behaviour, see this announcement:

http://forums.earthempires.com/Forum.php?threadid=3446

very interesting read, i'd forgotten of that

but still his destroyed building to unused acre causes a drop in pm, too?

Oh yea, I totally forgot about that!

Uh destroyed building I don't know, sure a turn didn't elapse?

Must have been looking at the wrong column. Just unused land then.

Didn't find this information in the wiki or game but good to know, thanks :).