No, you can not buy the ticket at Pingyao instead of Taiyuan. If you not go around festivals and holidays, the ticket is easy to get right when you arrive at Taiyuan from Pingyao.

yes you can just buy them at the station in taiyuan, thats what i do whenever i go to beijing! had no problems yet! just get there an hour before the train leaves...they like to change what platform you are meant to be at so make sure you have the right one and have some food with you, it takes 3 hours to get to beijing, usually beijing west! they serve food on the train but its expensive compared to china prices...like 6rmb for a bottle of green tea! enjoy!

Matt