Without power or an internet connection you can't validate that the btc hash is unspent, avoid double spends, or validate that your taking ownership of it made it to the blockchain and was confirmed for a few blocks.
Sure you can transfer the private key from one device to another, but (a) you can't know the other person didn't retain a copy of it and (b) you would be limited to spending the exact amount you have in an existing transaction because you couldn't send a transaction to the chain that splits it.
You've to got a reputation system with identities also stored on the blockchain, and you accept the transactions only from the most trustworthy people. Counterfeit paper currency exists, and counterfeit bitcoin (not BTC) transactions might exist, but not every person is equally suspect of using counterfeit money.
Sure, but then you aren't really using bitcoin. The whole point of bitcoin was to be a trust less system where transactions are verified cryptographically on the blockchain.
Transacting bitcoin private keys without going to the network and trusting the other party to not scam you defeats the whole purpose.
Sure you can transfer the private key from one device to another, but (a) you can't know the other person didn't retain a copy of it and (b) you would be limited to spending the exact amount you have in an existing transaction because you couldn't send a transaction to the chain that splits it.