The answer to this question comes with basic common sense and logical thinking:
Why do players sell combination coupons at all? They can be bought in the shop for 6.5Million, so why would anyone buy from another player?
The answer to why anyone would sell them is because they happen to have unneeded coupons that would make better use as a giga value, and selling to the shop will not be sufficient. People buy from another player to save gigas instead of spending it in the shop.
Supply and demand. If players are to sell combination coupons, they must sell it for less than a given player can buy from the shop. Therefore, it's not "how can people sell", it's more "why do people sell", a question I specifically pointed out.