For-sale signs have increasingly popped up since the spring on lawns of homes in the city, where a strong buyer's market has been created by an influx of new rental housing that real estate agents describe as “overbuilt.” Agents say houses have been much harder to sell in the buyer's market, as civilian and military families have a broader menu of housing options from which to choose by virtue of new apartment complexes. They say some families have been enticed by move-in specials offered by complexes, which have offered free rent and other perks.