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Is there a way to transfer/import partial amounts of BTC from one wallet to another off the chain in order to avoid transaction fees? I assume this is how services like Coinbase get around letting you transfer funds from your Coinbase to GDAX wallet without any transaction fee since they know both private keys. Is this possible? If so, how would this be done with something like bitcoinjs-lib?
That sounds like it's not possible at the moment. I can also create 1 wallet with 2 adresses. Then, I have also 2 private keys, but I still have to do a blockchain-transaction. How do you know that they don't pay fees? – ndsvw – 2017-12-30T22:43:35.387
They may internally pay the fees but I know that this cost is not pushed onto the customer. If that were the case how would they handle customers that move funds back and forth between their Coinbase and GDAX wallets? https://support.gdax.com/customer/en/portal/articles/2430129-transferring-funds-between-gdax-and-coinbase
Perhaps these aren't real "wallets" and just masquerading as such, since the service doesn't expose the private keys to any of their accounts.