My suggestion is that if you sell a coin item you will get coins instead of nothing.
A few problems have happened in the past. I accidentally bought a 1 year merc pack of Echo Force when I was suppost to buy an Arsenal 1 year. I had to sell it to the shop because no one would buy it. I didn't get anything. No gigas and no coins. If I had at least got half the coins of Echo force one year,that would have saved me a lot of time coin grinding. I know this is possible.