The opportunity to own a piece of Manhattan at a discount is vanishing as thousands of Americans jump in to buy real estate, boosting the struggling New York City market. More than 2,000 properties were sold in the first quarter of 2021, up 6.6 percent from the year-earlier period, according to a report by the New York City brokerage Serhant. Sales could be expected to rebound further as nearly 4,000 contracts were signed in the 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 statement. Part of the boom comes from “pent-up demand” from customers who wanted to buy, but put the plan off …