It takes time to find someone willing to part with what you want and even longer to find something of equal value in return. But that’s just fine for hundreds of locals who have joined Petaluma Trading Post, a Facebook group committed to the ancient form of payment. Started last month by Heidi Margocsy, a 39-year-old Petaluma photographer, the group has grown to more than 300 members, who have traded homegrown veggies, baby clothes, books and many other items with fellow Petalumans.