Buying:
Each virtual stock will be bound with a bitcoin address for buying. To buy X shares with Y BTC each, please send X * Y + X * 0.00000001 BTC to the buying address. X cannot exceed 9999 and Y has to be dividable by 0.0001. For example, 10.00000100 represents to by 100 shares with 0.1 BTC each. The sending address is called 'buyer's address'.
This one-size-fits-all would not do for most stocks. They really should reserve 3 bits (8 Satoshis) for a protocol version. For instance the published protocol would be modified as follows: buy by sending X*Y + (8*X+V)*.00000001 where V is the version number 0-7. Y has to be divisible by 0.0008 instead.
Now you can have up 8 versions running at once (reasonably 4 to give yourself flexibility to evolve). Maybe some of them would not require you to send 100 BTC to sell your shares.
... but this really needs it's own thread ...