At some point
- By some point, the marginal utility of these commodities had changed and the consumers stopped buying them at the current price.
I'm honestly not sure about this- it sounds weird to me.
- The consumers had hardly got(ten) the computers in surplus when the demand for them fell.
I wouldn't use the progressive form for 'rate'
- I had been rating meat very highly for a long time before I became a vegetarian and acquired a taste for fruit and vegetables.
- The members of the meeting had been discussing the problem of price controls and ration stamps for two hours before they came to the conclusion that it was the only way to handle temporary excesses of goods
The 'his;' doesn't refer to anyone- the more someone buys, the less his desire...
- Scientific research confirms the fact that the more units of a (or the?) commodity buys the less his desire to have this commodity gets/grows/becomes (do all the three verbs in italics sound good here?). Economists call this tendency “The Law of Diminishing Marginal Utility”.
The first is essential, IMO. You could keep or delete the ther two.
- –In my opinion, a variety of consumer goods on the market makes the problem of the choice of the commodity we need more complex. (the number of 'the' confuses me)
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