Apartment inventory in the Manhattan sales market is at its lowest point in roughly 15 years according to a market report released by real estate group Douglas Elliman today.

Buyers acted quickly to invest in 2013 ahead of expected rising interest rates in 2014, Douglas Elliman president and CEO Dottie Herman said, which resulted in the fourth quarter of 2013 having the most sales in the Manhattan market in 25 years.

"In the spring there was a lot of talk about interest rates rising in 2014, so all those spring buyers were going around like 'we better make a move this year,'" Herman said.

Low inventory doesn't mean new buyers shouldn't bother looking for apartments. But they should come prepared.

"I would say that if you are looking for apartments in the city you need to know what you're looking for, so that when you find something you really have to act quickly," Herman explained. "You won't have time to think about it."

While it may sound like more Manhattanites are living on top of each other, the low inventory is actually another sign of the national economy improving.

Prices are also rising, with the median sales price in Manhattan up 2.1% to $855,0000, according to Douglas Elliman.