can i buy bus ticket in advance from putuoshan back to hangzhou

I am afraid tthat the online reservation service is not available. Just buy a bus ticket on the spot. Usually there will be tickets left upon arrival. No worry :)

may i know how long is the journey from putuoshan to ningbo railway station. Please advise how to go about it the faster and easy way. thank you

Well, you can take the coach from Putuo Central Bus Station and arrive at Ningbo South Bus Station first. The trip takes around 2.5 hours. It costs about CNY 50 per person.
After arriving, you can walk south around 220 yards (200 meters) to get to Ningbo Railway Station.