It is possible in China to buy most tickets from other stations, as much of the ticketing system is networked already.
Let’s say you’re in Zhengzhou, central China, and you want a ticket from Beijing South to Shanghai Hongqiao (HSR train). Most stations that offer networked ticketing will also allow you to buy a ticket from another station. Put it this way, a ticket from another station has to be — as the name states — a ticket offering you a ride from place A to place B, whereby none of these places are where you are presently located in.
Often, when you buy one of these tickets, you’ll have to pay RMB 5.— extra for the service, but then you also get a receipt. You can keep this as a souvenir; you won’t actually need to show this to get on your train.
If you’re a PRC citizen with a valid ID card, you can actually legally avoid this RMB 5.— service charge by getting your ticket from machines, or by using your ID card as an e-ticket (provided you booked online). However, for holders of other forms of ID, you’ll have to use the counters to get your tickets.