Rinkeby Testnet – Resolving Transactions Not Going Through

I still don't know why eth.resend was failing (and continues to fail as of geth 1.6.5), but this compatible patch works for me:

eth.resend = function (tx, gasPrice, gas) {
  if (gasPrice) {
    tx.gasPrice = gasPrice;
  }
  if (gas) {
    tx.gas = gas;
  }
  tx.data = tx.input;
  return eth.sendTransaction(tx);
};

I prefer to also add this convenience version:

eth.resendgwei = function (tx, gasPriceInGwei, gas) {
  if (gasPriceInGwei) {
    return eth.resend(tx, web3.toWei(gasPriceInGwei, 'gwei'), gas);
  }
  else {
    return eth.resend(tx, null, gas);
  }
};

Now, if your pending transaction is stuck because the gas price is too low, you can speed it up with:

eth.resendgwei(eth.pendingTransactions[0], 27);

Have a look here Understanding transactions better

Full nodes listen and broadcast transactions. As a full node you receive and can display the pending transactions in your transaction pool. That's what etherscan does. Maybe you can't access it through their API, their choice.

So you can get some of the pending transactions as a node, but you don't get to choose, and most likely some transactions will have been included in a block before they reach you in a pending state.