The opportunity to own a piece of Manhattan at a discount is vanishing as thousands of Americans jumped on the buys, boosting the struggling market. Over 2,000 properties were sold in the first quarter of 2021, up 6.6 percent from the previous year, according to a report by the New York City brokerage Serhant. Sales could be expected to rebound further as there were nearly 4,000 contracts signed in the first quarter, the most since at least 2010 when Serhant started tracking the numbers. “While sales slowed at the end of March 2020, the fact that Q1 sales surpassed the pre-pandemic market last year is nothing short of phenomenal, considering the unique impact the coronavirus had on Manhattan,” said Ryan Serhant, the firm’s chief executive, in a release. Part of the boom comes from “pent up demand” from customers who wanted to buy, but put the plan off due to …