Why do the chinese count with 正?
I'd like to know why that particular character is used. (I have seen it used in stamps before too).


because 


正 is a common character
正 has five stokes
All five stokes are straight and no overlap, so it is easier to count each stoke.
The character neatly form a square shape, make counting multiple copies easier
The stoke order and stoke type are simple . It just repeat two different straight stokes: 


Left to right horizontal straight line 
Top to bottom vertical straight line



One can write 正 faster than most other five stokes characters