Why not make the locker, the transfer service as well. Rather than maintaining and monitoring two sources (user and recipient computers) make the service an online locker so that transferring songs is really just a database update? Allowing the locker service to only maintain 1 copy of a file, but allow access to many people, rather than "transferring" said file.
The service could have streaming as well via web or app.
I'm sure that opens up an entirely new can of worms though.
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