people selling stuff second-hand seems like the LEAST insidious aspect of instagram. like taylor says private listings will happen anywhere regardless. the REAL "marketplace" effect of the platform is the commodification of the music itself, and the reduction of creative impulses down to whatever meme-wave happens to be passing through the public consciousness at the time. all aesthetics become interchangeable, seeing black metal font does not mean black metal anymore, seeing "b/w pictures of guys in ruins" could be industrial, or techno, or some graffiti street artist, or anything. so much information REDUCES peoples' ability to conceive really new things. there are too many points of reference.
edit: for me, i can't type the names of some of my projects on instagram because they get immediately flagged ("controlled opposition," "weird dick,") it's an interesting challenge :)