The number of homes listed for sale grew in many US cities in June, a month that typically brings a slowdown in listings. Inventory grew amid signs that demand plunged after the expiration of the home-buyer tax credit. The supply of homes available for sale in 27 major metropolitan areas at the end of June was up 3.7% from one month earlier, according to figures compiled by ZipRealty Inc., a real-estate brokerage firm based in Emeryville, Calif. The data includes all single-family homes, condominiums and townhouses listed on local multiple-listing services in markets where the firm operates.