to buy 100 shares i just need to do
bmc.py buy 100 SIN 1
correct?
You need to do:
bmc.py buy 100 SIN 100000000
The GLBSE client doesn't use decimal points yet, so 1 BTC is entered as "100000000". If you type:
bmc.py buy 100 SIN 1
that would actually be entering a buy order for 100 shares at 0.00000001 BTC.
For more info and examples, check out the "Numbers" and "Buying" sections of the tutorial:
https://gitorious.org/black-market/pages/Tutorial