itch.io is the best! Meets basically all of my wants: DRM-free, open source client that isn't required (unlike GOG's client, that also isn't available on Linux), and I assume there's a refund system although I haven't looked into how good it is in comparison to Steam/GOG.
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For me anyway it isn't just the vagueness of their return policy but the effort it requires. On Steam I can simply click a button for a refund and the process is started. I don't have to explain anything or talk with anyone. For Itch.io you can request a refund but it requires explaining to someone why you need it. I do understand with stores with Itch it isn't as easy as Steam due to the games being DRM free. In theory with too easy of a refund policy one could buy the game, download the install files and then refund it while keeping the files. With Steam since you can't download the install files you can't do that as easily.