Many of your questions would equally apply to Bitcoin. For example, your question about transferring $10 to person A applies to Bitcoin as well. Ethereum uses

Many of your questions would equally apply to Bitcoin. For example, your question about transferring $10 to person A applies to Bitcoin as well. Ethereum uses a blockchain, as pioneered by Bitcoin to ensure that transactions are not spent twice. For your last example, you might put the $10 in escrow, with a key held by you, one by the book owner, and one by a trusted third party. If you lose the book, if the third party and book owner agree, your money is released from escrow.