Which bus should I take from CQ to Youyang? Can I buy a bus ticket right before departure?

You can take bus ticket right before departure. But there is a bus from Chaotianmen coach station to Youyang every 1.5 hours. So, you may have to wait for a while if you miss the previous one. Trains to youyang are from Chongqing North Station and the fastest train (K9461) takes 4 hours to Youyang. I would suggest you buy a sofe seat of the train.