I'll make a trip from Hangzhou to Wuzhen

From 07:00 to 18:20, there are 34 buses available. Nearly every 25 – 30 mins, there is a bus departing. You don’t need to buy the tickets in advance, but purchase it upon arrival. The bus schedule is dense, so you can catch one easily. I’m afraid you cannot buy the retuning tickets in the bus station, well booking online is feasible.