On trade, What justificaion does the USA have to accuse China of 'selling too much not buying enough from USA' There's a trade war (dispute) going on between USA and China, and it boils down to the USA accusing China of not buying enough American products, relative to what they sell to America. 

Putting aside other accusations such as currency manipulation, cyber security hacking, etc. If we just focus on this central accusation of 'You are not buying enough of my stuff, while selling too much stuff here!"

I don't understand how that can be a valid 'accusation'. Buying and selling are willing actions taken by market participants, made as a result of their own economic situation. China has total freedom to decide what and how much to buy (import) from any country, as does the USA. If China decides not to buy certain products it probably means either it thinks those products are too expensive, too low quality, or maybe China doesn't need them. Same with USA - it decides to buy certain products from China at certain amount because it has decided it is the right product at the right price and amount. My point is, USA can decide to buy however much from China and vice versa. These are willing market participants.

If so, how can any country 'accuse' of another of not buying enough or selling too much? Whatever China sells is agreed to be bought by USA, it's not like China forces USA buy its stuff. And whatever China buys from USA is its own decision.
The US president see’s trade as a zero-sum game, where there is always a winner and a loser, being the winner the one who sells more to the other. This view is not shared by most economists, as the mainstream opinion is that international trade is beneficial for both parties (see Ricardo or Krugman for examples). 

In the US’s president mind, “to win the game” they must restrict the sales of Chinese goods, using tarifs or quotas. The US can either produce these goods or import them elsewhere. The Chinese, however, are expected to have no other alternative but to keep importing advanced goods, critical for their economy, such as aeroplanes parts from Boeing. 

The expected outcome from this policy is less competition for national producers, potentially resulting in more jobs and higher profits. However, end consumers might end up facing higher prices for several goods, resulting in lower real incomes.