Final answer:
The word 'influx' is used correctly in the sentence to describe a large number of people going to the grocery store due to a sale.
Step-by-step explanation:
The vocabulary word influx means a flowing or coming in of something or people, often in large numbers. Therefore, the sentence 'There was an influx of people at the grocery store after a huge sale was posted in the local newspaper' uses the word 'influx' correctly. The use of 'influx' indicates that a significant number of people entered or arrived at the grocery store in response to the advertised sale.