Hi.how to buy a bus ticket from xiamen to macao?

There is no direct buses to Macau from XM.
You need to go to Zhuhai first and then go to pass the Gongbei border to reach Macau.
To reach Zhuhai, you can fly to reach or take long-distance buses from Hubin South Bus Station.
The bus takes aroun 12 hours and the plane takes around an hour and twenty minutes.
Buying a bus ticket, the ID card or passport is not needed.