Data point:
Just had a very successful churn at my local GS (Reading/Berks County area). I've rotated through 5 different stores, and this is the first time I was recognized. It was the store manager. He was extremely helpful and said something to the effect of:
"Hey I've seen you coming in here buying the eBay cards with your gift cards. Just as an FYI, if you trade in games, devices, etc. you will get a voucher. That voucher can also be used on gift cards. And not just eBay, either. You can use it on
Amazon, google play, and anything else on that rack. You also continue to get your reward points as well."
So, not only was it allowed here, but I was encouraged to use my rewards every time, use my vouchers for GC's, and pointed out that I could use them on Amazon. Before he said that, I did look and verify that The Kindle cards were still on the rack. They were.
I also drove to my local Sears. The cashier recognized me too (I had her last time). She asked about the use for the Disney cards. I explained how they can be used to pay for vacation, and anything on property, the discounted ones at Sam's Club, the savings with the vacation account, and the security of being able to transfer balances if one card is lost. She was impressed and thought it was a great idea.
I conveniently left out the churning process.