Why do the chinese count with 正? I have seen a couple of shop keepers (in small - mid sized shops) in Hong Kong counting some routine things with this character : "正". The actual tally would start off with the first stoke of that character and increase by 1 according to the stroke order of the character. So, one 正would effectively represent 5 people.

I believe the Western tally also counts to a maximum of 5 per tally.

I'd like to know why that particular character is used. (I have seen it used in stamps before too).
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