New York Attorney-General Eric Schneiderman has ordered an investigation into complaints of prices being increased in the aftermath of Sandy.
Mr Schneiderman said the largest number of complaints concerned increased fuel prices, but other emergency supplies were also affected.
The BBC reports more than one million people in New Jersey and New York City were still without power on Monday, a week after the storm came ashore near Atlantic City.
In a statement, Mr Schneiderman said that under New York state law, retailers were not allowed to charge "unconscionably excessive prices" for goods required for personal, family or household purposes when there was an abnormal disruption of the market.
He pledged to do "everything we can to stop unscrupulous individuals from taking advantage of New Yorkers trying to rebuild their lives".
The BBC reports more subway lines were opened on Monday, as more commuters returned to work and one million students returned to school for the first time since the storm.
Queues formed early on Monday for the Jersey City ferry service to New York.