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The rent book feature is not available right now, but we are thinking of using SBD as the currency of exchange. We will fine tune the idea before realising the feature. Suggestions are welcome please.

It would be great to be able to pay by upvoting the publication somehow.

The flunctuations in the price of steem will be a major hindrance to the upvote payment. After the 7 day period has lapsed, there's every possibility that the value of the vote might be less than the worth of the book, that's a loss to the writer. It could end up being more than the worth also. I feel the best way to handle payments will be through transfer of funds (SBD/steem)