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Could this be built for steem by a community (blockchain) dev initiative?

Or is it really only in Steemit Inc (or maybe Blocktrades) territory?

delegations already exist, and there is existing rental markets.... why do we need rex?

In theory, anything could be built by a dedicated community. It's all open source software.

I am guessing it would require a hardfork to implement?

Maybe. Not sure. Resource credits are already a thing. Maybe they'd just need functions for transferring them between accounts.