Why is this allowed?

This user is listing over 100 cards a day at price that is over the real price of the specific item. For example he puts $10 google play cards for 11, 12 usd and with the fee the card comes around 13 usd for me ( the buyer )

I thought gameflip is to save money not to overpay

Well, he is not doing anything wrong. If he has this amount of cards he can sell at his own price. Probably is not worth for you to buy, but no one force you to do this.

Seller can sell his own items on his own prices.

Gameflip is marketplace, not store so user can make they own prices.