Why would someone not simply sell their GH/s from cex and then withdraw the bitcoin and then sell the bitcoin on an exchange or on the currency exchange section?
For btc-e why would someone not sell their bitcoin on their platform and then withdraw the fiat to a payment method of their choice?
1. Selling btc in the exchange section or codes here is fundamentally the same. If you sell the codes you save money on commission and spread.
Are you offering a 1:1 price for these codes? Your OP fails to mention the price you are offering, however most people who have listing on this forum tend to charge a spread and this spread is often steep, and gets steeper as people try to sell things that are less then "true" bitcoin, that is bitcoin to an address they 100% control.
2. If you withdraw fiat from CEX.IO or BTC-e it will take 2/3 working days best case. CEX.IO can take up to 20 working days.
It is not guaranteed that you would be able to send someone their money the same day. (explanation continued below)
3. Clearly you never tried to withdraw FIAT currency from BTC-e. Give it a try. Minimum fee is 100 EUR
You are correct. If you are in a country that coinbase supports then you can withdraw to your bank account for what is essentially free (you would still need to pay their spread of ~.5% but this is less then the spread that most people on here will charge)