Coinbase

It’s important to understand how Coinbase handles digital currency transactions to ensure any refunds reach the sender’s account. Coinbase does not support digital currency being returned directly to an address it was sent “from” (in the digital currency world, we call these input addresses). 

When a Coinbase user sends digital currency from their wallet, the address it comes "from" is one of Coinbase's many hot wallet addresses. Any coins sent back to that address would be sent to Coinbase, not your own wallet.

For refunds sent back to a Coinbase customer, there are several ways you can go about obtaining an address which will accept refunds for a particular transaction.
    

1. You can simply send your refund back to the sender’s registered Coinbase email address, and our system will match this to their account.
2. You can request they log in, navigate to a digital currency wallet of their choice, and generate a new address using the “My address” button near the upper right.
3. You can request the sender provide you with an address from https://www.coinbase.com/addresses while logged in to their Coinbase account. Each address here is capable of receiving and matching a refund.

Once you have a digital currency or email address provided for refund, use the send page, include the address provided, enter the amount to be refunded, and click 'Send Funds'.

If the party you are trying to refund is not using Coinbase, you will need to reach out to them directly in order to obtain an address that can accept refunds.

More information on Coinbase transactions is included in the following article: 
Why can't I see my transaction in the blockchain?