How to retrieve readable private key from bitcoinjs-lib
use .toString('hex') instead.


  
  Don't export / import individual private keys when you are using BIP32 HD keys. IT IS DANGEROUS.
  .toString('hex') and .toString('base64') are readable string representations of Buffers, but no crypto apps use them. The accepted format for individual private keys are WIF. But like I said, IF YOU ARE USING HD KEYS, ONLY STORE/EXPORT/IMPORT YOUR SEEDPHRASE OR AN XPUB.
  


Source: From this Github thread.