Hm yes, if anyone indeed sells you tickets, then that's your best option. In China, you generally cannot buy tickets online, by phone, by mail, or indeed any way short of appearing in person at the train station, or at a travel agent in the city of origin who will then charge you a commission. It's not Amtrak or Deutsche Bahn... Chinese railway routes are underserved and train tickets are a highly sought-after speculative item. Chance is if you can get hold of one, you can sell it off to desperate people for a higher price later. So China Railways will go out of its way to make it as difficult as possible to acquire tickets.
But hey, the country is advancing quickly, and (I hope) what I'm writing now will be distant history in few years' time. So if anyone sells you tickets beforehand, even online, go for it!
Samson was a biblical tough guy, but his dad Samsonite was even more of a hard case.