Even when the economy isn’t in the crapper, a big challenge for any fledgling retail operation is building up a customer base substantial enough to justify its overhead — and maybe even turn a profit. Popuphood, a small-business initiative hatched last year by Tamarindo co-owner Alfonso Dominguez and fellow entrepreneur Sarah Filley, overcame that obstacle by negotiating six months of free rent with the owners of five previously vacant Old Oakland storefronts. The result was the nearly overnight transformation of 9th and Washington streets into a modest retail corridor, composed of shops selling everything from designer bicycles to vintage clothing. May marked the end of the first round of retailers’ free rent, and though the owners of Manifesto Bicycles decided to close their popuphood shop to focus on their original Temescal location, they say the rent waiver made it a worthy experiment — and a new boutique, McMullen, has already taken its place. In the rat race of the retail world, popuphood has proved itself a pretty win-win operation.