After plunging throughout 2012 and for much of 2013, and rising only modestly through the beginning of this year, the inventory of all for-sale homes nationwide spiked in May, jumping 11.8 percent year-over-year according to a new report from real estate portal Zillow.

In the Boston metro area, inventory of all for-sale homes fell 6.2 percent year-over-year but rose 7.4 percent month-over-month in May, the first monthly gain so far this year. The lowest-priced homes face the largest decrease in inventory, down 0.8 percent, followed by the middle tier, which was down 0.7 percent. The number of homes for sale in the highest price tier was up 1.6 percent year-over-year in May.

That echoes the pattern nationwide, where most of those gains in inventory were made among homes priced in the middle and top one-third of home values. The number of homes available for sale in the most affordable price bracket, those homes most sought by first-time homebuyers, fell year-over-year in 28 of the nation’s largest metro areas analyzed by Zillow.

The total number of homes listed for sale on Zillow in May was up 4.3 percent over April, and has risen month-over-month in each of the past three months on a seasonally adjusted basis. Overall inventory of for-sale homes was up year-over-year in 506 (78 percent) of the more than 600 metro areas analyzed by Zillow. Large metros where inventory has increased the most include Las Vegas (up 51.5 percent year-over-year), Washington, DC (up 45.7 percent year-over-year) and Riverside, Calif. (up 42.7 percent year-over-year).

"It’s good to see overall inventory rising. It’s likely that many would-be sellers have decided to capitalize on recent home value gains, particularly as the pace slows, and list their home for sale now in order to move into a new home while mortgage interest rates remain low," Zillow Chief Economist Stan Humphries said in a statement."But persistent inventory constraints at the low end of the market continue to make it a tough environment for first-time and lower-income homebuyers."

In addition to low numbers of affordable homes for sale, first-time and lower-income homebuyers armed with traditional financing are also competing with all-cash buyers at the lower end of the market. In 27 of the top 30 metros analyzed by Zillow, more than one third of all sales of the lowest-priced homes were made with cash. In three of the top 30 metros – Tampa, Detroit and Miami – more than 80 percent of all sales in the lowest price bracket were cash deals.