Amazon already has customers. If their other products are to go by, they'll just give you an account if you have an Amazon account. Probably combined with free games if you have a prime account and you can imagine that it won't take much to compete, at least not for a company like Amazon.
That was my thought. They already give away free games over Prime, if they leverage that they have already given a large number of people stake in their new market place. Plus they own Twitch, I don't believe there is a publisher who isn't interested in the idea of people being able to impulse buy whatever there favorite streamer is playing without even leaving the stream. The strategy is pretty easy actually, give streamers a cut of each sale and encourage them to put up notifications when it happens like they do subs and cheers.
The free games on prime accounts is probably exactly what will happen, and will probably be what needs to happen for it to be any amount of successful.
Look at Epic which offers free games but sees pretty slow growth outside of their flagships. Further, look at Amazon's lumberyard engine, which gathers dust for the most part.
I'm not convinced that their 'weight' will automatically guarantee wide adoption.
