Import private key to coinbase? Is there a way to import a private key that I have been using in bitcoin-qt to my coinbase wallet?
Coinbase no longer supports any sort of wallet import (wonder why?). To quote Coinbase (June 2016 on https://support.coinbase.com/customer/en/portal/articles/2285419-how-do-i-import-an-existing-wallet-)


  Coinbase wallets currently do not support a wallet import feature. If you have paper wallets, private keys, wallet.dat files, or brain wallets, they can be imported into another wallet (Bitcoin QT, Electrum, Multibit, Armory, Blockchain.info, etc) before being sent to your Coinbase wallet.
  After you have imported your wallet, you can keep your bitcoin there, or send it to one of your Coinbase wallet addresses.