If Instagram and eBay had a hip younger sister, it would be Depop. It works just like any other selling site or app, but seeing as it's used mainly by under-30s, the imagery of the items you want to sell need more treatment, much like you'd give those on your Instagram feed, as the demographic expect as such. Users can like and share your pieces, too.
Pro tip: You can often command a higher price on Depop than the likes of Gumtree, so aim high, and stipulate if you're open to offers.