Might try the developers forum, although I don't know what rules Roku might have on marketing something on their forums. Better look at the forum rules first. Outside of that, I can't say I've ever heard of such a marketplace. Would it work on EBay? I really have no idea.
Nvidia Shield, Roku Stick (3600), Ultra (4640), Premiere (3920), Insignia 720p Roku TV, Sharp 4K Roku TV, Windows 10 Pro x64 running Serviio and Plex on a wired Gigabit network.