I wouldn't recommend doing that either. While I think the hotel suggestion is the best, it's worth noting that most Chinese cities have train ticket windows scattered about; there are 3 such locations within 500m of my place in Beijing; buying from them is no different than buying from the ticket office at the train station itself (in fact, if detune yourself from your surroundings, you actually feel like you are at a train station).

I checked out tripadvisor once and was slightly put off by their service charges (much more than even the most expensive hotels).

In regards to the earlier mention of 无座 tickets, it is worth noting that at many, if not most, midpoint stations, you can't buy anything else. However, upgrades to soft sleeper tend to be pretty easy to snag for those with enough determination (most Chinese fight over the hard sleepers, leaving the soft sleepers for the taking).

For anyone that's interested, I explained the upgrade process to the best of my knowledge in my recent xinjiang trip report; searching that thread (and sorting by post) using the term "upgrade" should get you to the right place.