Firstly in relation to online selling:

Sellers who offer goods or services online typically require payment before delivery and for instant services like downloading digital content, it makes sense to accept LN for instant settlement. For physical goods requiring shipment, it is reasonable to ask for on-chain payment if preferred and Sellers could confirm transaction before shipping.

Secondly in relation to purchase of book with LN vs dirty fiat: If a seller demonstrates commitment to the circular economy and BTC adoption by accepting BTC for buying goods and services and using for their own purchases, I would be inclined to support them with sats. However, if we do not appear to share similar values this may also be opportunity to unload my dirty fiat in exchange for a book I want anyway.

My preference would be to do business with a seller that I also have shared values around adoption and moving towards a Bitcoin standard. If you not a fully orange pilled sovereign maxi …No sats for you! LOL