It's a seller's market in Nassau County.

At the current pace, it would take just 5.8 months to sell all the homes listed for sale in the county, according to September data released Friday by the Multiple Listing Service of Long Island.

That's down from 6.5 months a year earlier and marks the third straight month that inventory has lagged below a six-month supply of homes.

A balanced real estate market has a six- to eight-month supply, brokers say. More than that, and buyers are in the driver's seat.

"The seller has the upper hand," said Debra Gusler Walsh, a real estate agent with Petrey Real Estate in Long Beach. "I tell buyers all the time, 'As soon as you want it, someone else does, too.' "

In one instance, a 480-square-foot, one-bedroom condominium in Long Beach that was listed at $239,000 ended up selling for $250,000 after multiple bids were received, she said.

The competition among buyers pushed the Nassau median home price up by 2.8 percent year-over-year, to $440,000. Buyers closed on 1,176 sales in the county in September, up 5.8 percent from a year earlier. Brokers listed 6,785 homes.

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In Suffolk County, buyers still hold the edge, MLS data shows. Suffolk's supply of homes fell by 0.5 percent, to 10,616, but there's still an 8.6-month supply. In some sections of Suffolk County, the real estate market remains bogged down by foreclosures and vacant properties, Walsh said.

Suffolk homes fetched a median price of $335,000, up 1.5 percent from the previous September, and the number of home sales increased by 1.5 percent annually, to 1,236.

Before Nassau switched to a seller's market in July, it had been either balanced or leaning in favor of buyers for almost two years. From September 2013 through June, supply ranged from six to nearly 11 months. In October 2010, it hit its most recent peak of 16.5 months.

Suffolk has not had less than six months of inventory since March 2005. In July 2010, supply peaked at 21 months.

In both counties, nearly three years after superstorm Sandy battered Long Island, buyers no longer fear buying coastal properties, Walsh said.

"After Sandy I thought I was going to have no job; I thought, 'How am I going to survive?' " she said. Now "I am busier than I've ever been."