If you want to sell your Ethereum from paper wallet, first you will need to import your paper wallet into Ethereum wallet/Coinbase Wallet.
I've never used Coinbase Paper Wallet, but from my understanding you will need to use your Coinbase Key on your Coinbase Account, if done successfully you should be able to recover your Ethereum inside your Coinbase Account, if you don't know how to recover the ethereum you can read "Recovery Instructions" Section.
If you want to sell them, you will need to send those ethereums into an exchange such as Bitfinex, Hitbtc. etc
Other option is to use Coinbase instant exchange
https://support.coinbase.com/customer/en/portal/articles/2021569-instant-exchangeMy Knowledge regarding Coinbase are limited as I don't use them anymore, so others might have more decisive answers than mine