Is there a difference between 买 (mǎi), 购 (gòu), and 购买 (gòumǎi) which mean "to buy"? I'm trying to parse this sign I saw at Beijing airport when catching the bus to Tianjin.  The sign indicates where to buy the bus ticket.




  清您先到13号门内省际购票厅购票


The hanzi 购 is confusing me.  I'm now aware of three ways of saying "to buy":


买 (mǎi), which arises very early when learning Chinese,
购 (gòu), and
购买 (gòumǎi).


Question: Is there a difference between 买 (mǎi), 购 (gòu), and 购买 (gòumǎi) which mean "to buy"?
买 is more casual. 

购 is formal when forming up with other characters: 购买，购票厅，购物，申购，团购，and etc.. These terms are formal both in written and spoken.

You can also consider 买 is short for 购买. 购买 is official term, so people just say 买... in our daily life instead of 购买. For example, 买菜，买车，买房子.

As this sign is posted in the Beijing airport, they should make it formal by 购票厅, instead of 买票厅.