When Rasheeda Gray was hunting for her first home in Philadelphia, it seemed as if her options were endless.

"Back in 2006, I think we looked around at 30 or 40 homes in a month," said Gray, now 34.

At the time, she was 24 and a recent college graduate. Yet she was ready to buy - a mortgage at the time was cheaper than rent for her - so she and her husband began searching. It was the peak of the boom, and mortgage rates were high.

Home prices, too. But nearly 3.5 million homes were on the market, so when Gray and her husband were excited to move quickly, they had options.

When the couple, now with kids, began looking to upgrade to a new home in summer 2015, they immediately noticed one thing - fewer for-sale signs standing in front yards.

Even as the housing market has been recovering, at least one big issue has remained, observers say: low inventory, meaning that there are not enough homes on the market to meet buyer demand.

For sellers, low inventory can be a good thing. As eager buyers scramble to lock down a limited number of homes, sellers can often wait for the best offer - many times, it's the asking price, or higher.

For buyers, however, low inventory can lead to frustration. Bidding wars are not unusual - guides abound offering instructions on how to win one - and many buyers are stuck shuffling from listing to listing.

Where that has left the market? In third-quarter 2016, the Philadelphia metro area had about 7,400 listings, a 15 percent reduction from the year before, according to Trulia. Nationally in December, about 1.65 million homes were on the market, according to the National Association of Realtors - a 3.6-month supply.

A "healthy" supply - expressed as the time it would take all homes on the market to sell at the current pace - is about six months.

How did the market get here?

For starters, the U.S. economy is improving. In January, 227,000 jobs were added, outpacing expectations. Wages ticked up slightly.

Buyers who feel financially confident are flooding the market, so "when a decent house comes online at a good price, there are probably one or two or three buyers interested," said Danielle Hale, managing director of housing research at the Realtors Association.

Meanwhile, new-home construction remains low. During and after the recession, construction employment crumbled, with more than two million jobs lost. For those builders that remained, tighter lending standards made it harder to get financing for new projects.

"We have not seen a lot of new construction after the housing boom and bust," Hale said.

Plus, she noted, "There's a bit of a mismatch between what builders want to build and what buyers want to buy."

Without a doubt, an increasing number of builders have been focusing on higher-tier homes, where bigger profits exist. According to Zillow, in September 2016, more than 600,000 top-tier homes were available on the national market. Bottom and middle-tier homes, meanwhile, hovered around the 300,000 mark, Zillow found.

"It makes it difficult for entry-level buyers to get in," Hale said.

When Gray, a communications executive, and her husband were house-hunting again in 2015, they were looking for moderately priced homes. But unlike her first experience in 2006 - when they purchased a twin in Olney - there were fewer properties available to tour.

Seeking a place closer to family and with a strong school district, Gray was largely looking at Cheltenham, she said.

So weeks in, when they found a previously foreclosed fixer-upper for $206,000, they pounced. It had been on the market for only days, but, Gray said, "I knew we had to jump."

Luckily, she added, "I think we will be here for some time."

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