Sentence:  Such payments shall be made to small domestic producers of zinc as long as the market price for prime western zinc at East Saint Louis , Illinois , as determined by the Secretary , is below 14 - 1 2 cents per pound , and such payments shall be 55 per centum of the difference between 14 - 1 2 cents per pound and the average market price for the month in which the <WSD>sale</WSD> occurred as determined by the Secretary . 
Meaning of the word marked with <WSD> tags:  a particular instance of selling