Emily Useche, who is 27 and lives in Arkansas, had just put her baby down for a nap one afternoon when she decided to post some family photos on Facebook. But she didn’t simply upload them for friends and family to see.

She also posted the photos to a private Facebook group for a whole other community: A fan club for Pyne & Smith Clothiers. Ms. Useche was wearing one of that brand’s dresses in the photos — a style she had posted about once before when she saw it being sold secondhand — and was ready to show it off. Minutes after she posted, other members replied with compliments for her, and praise for the sunflower check dress she was wearing.

The group, Pyne & Smith Clothiers BST and Chat, is one of a number of so-called buy-sell-trade communities. Part social club and part marketplace, the groups have sprung up on Instagram and Facebook and have, for some users, become a daily place to socialize and shop.

While many serve enthusiasts of mass market brands, others are powered by dedicated followers of idiosyncratic indie brands, the sort rarely featured in glossy magazines and often escape the notice of major retailers. But they have devoted followers, many of whom are attracted by the idea of slow, ethical fashion.