In the guidelines which is linked in the footer of every page on Gameflip:
RANDOM ITEM LISTINGS
Any listing that is created without identifying the exact item that will be sold is prohibited on Gameflip.
Creating listings where the Buyer has a “Random”, “Chance”, or any sort of probability of items receive will fall under this. Listings that do not identify the item to be sold will be deleted and any accounts associated with the listing may be suspended or banned.
You can sell “crates” which are part of the game themselves, but you can’t sell stuff where you decide the chance. Post your invite code in case a moderator can help you with this.