Solidity: How to Use BalanceOf Function in ERC-20 for EthereumJS

Here's an example on how to do it from the web3 documentation:

var Tx = require('ethereumjs-tx');
var privateKey = new Buffer('xxxxxxxxxxx', 'hex')

var rawTx = {
  nonce: '0x00',
  gasPrice: '0x09184e72a000', 
  gasLimit: '0x2710',
  to: '0x0000000000000000000000000000000000000000', 
  value: '0x00', 
  data: '0x7f7465737432000000000000000000000000000000000000000000000000000000600057'
}

var tx = new Tx(rawTx);
tx.sign(privateKey);

var serializedTx = tx.serialize();

//console.log(serializedTx.toString('hex'));
//0xf889808609184e72a00082271094000000000000000000000000000000000000000080a47f74657374320000000000000000000000000000000000000000000000000000006000571ca08a8bbf888cfa37bbf0bb965423625641fc956967b81d12e23709cead01446075a01ce999b56a8a88504be365442ea61239198e23d1fce7d00fcfc5cd3b44b7215f

web3.eth.sendRawTransaction(serializedTx.toString('hex'), function(err, hash) {
  if (!err)
    console.log(hash); // "0x7f9fade1c0d57a7af66ab4ead79fade1c0d57a7af66ab4ead7c2c2eb7b11a91385"
});

SOURCE

The problem is that you are sending a non-zero value (1gwei in your example). If you want to transfer tokens then you have to set value to zero.

var rawTransaction = {
  "value": 0,
   ...
};

The reason is because for security functions that receive ether must have the payable modifier. Your transfer function doesn't have that modifier, and will reject any transaction with a non zero amount of ether.

function transfer(address _to, uint256 _value) public {
    _transfer(msg.sender, _to, _value);
}