Many international buyers get frustrated in sourcing from China's open market of miners. Let me help specify the current general terms and situations,

1. Generally, these transactions are between individuals, although the sellers may have some legal entities to register their sales people, paid 30% up front, and 70% before shipping out.
2. There are 3 parties on the selling side - the Miners who physically own the mining rigs; the Brokers who help Miners market/sell the mining rigs; the Repairers who help repair the broken miners. Usually, you talk with the Brokers who handle most transactions directly, because the Miners and Repairers generally do not want to talk with outsiders.
3. Before the Cryptocurrency totally crashes, when mining is still profitable, the Miners generally do not want to sell, because each day they keep the rigs in their hand, it's a net-profit for them. This was why I say now is actually a good time to sell, rather than buying mining rigs.
4. The most important fact - if you buy from a Broker now, it's highly possibly there are 30~50% of mining rigs are broken/unhealthy/not-enough-hashing power, that need immediate repair works. We already encountered 2 cases like this, and the only way to solve it is to repair - we cannot say the Broker is all bad, at least some responsible guys did help to repair.
5. If you are unsatisfied with a batch of goods, generally you have to cancel whole transaction and return the whole batch, or, you can wait for the broken units to be repaired. You cannot accept some units and return other units to save your time. This is another reason that we suggest our friends not to buy now, because we believe this “rule” is just generated from the current bull market, and it shall change later. 
6. If you buy from a Western country's online store, better make sure you really get to know below information,
   1. Who their bosses are?
   2. Can they repair?
   3. Do they have a right logistic team?
   4. Do they understand current open market situation in China?

Good luck, guys!

Morris, ICI - Network Expertise Helps You Succeed