Employees, directors or major shareholders buying/selling shares of their company is not considered as share buy back (SBB). It is insider trade......SGX has certain guidelines on this to ensure fair play to minorities......its totally different from SBB. 

Share buy back is when the company buys their own shares.......e.g Chuan Hup (the company) buys Chuan Hup shares from the open market. 

When Co. buys back their own shares, they usually do one of the following with it:

1. Cancel the buyback shares. This will in effect reduce the issued shares. Mgmt do this for different reasons. It could be to enhance the eps of the company. e.g. if YoY earnings are flat, when it is divided with a smaller number of issued shares, the eps will be higher after share buy back. Major shareholders can also use it as a tool to strengthen their grip on the company. e.g Chuan Hup....major shareholders (Peh father and son) only secured about 45%(???this figure may not be accurate) in their GO (general offer) attempt in 2007. But when CH completes its share buyback, the Pehs will effectively control about 48+% of the reduced issued shares.

2. Place them in a Treasury Account. These shares are not cancelled but  kept in 'hibernation' for reissue at a later date. (In 'hibernation' cos these treasury shares do not receive dividents). Treasury shares are reissued for executive stock options. This is a better way than issuing new shares for option cos its non dilutive in nature. Also, it helps the companies have a better cost control on executive stock options. New accounting regulations requires companies to treat stock options as expense. Share buy back give companies the flexibility of buying back when stock prices are low (reduce cost)........rather than expensing it when option is exercises.......this is usually done when price is high..