i wanna make a online game where u can sell stuff that others can buy, but im not sure what i should make the stuff avalible for selling in the game and what i can use to complete the game

i could try smth like fishing and selling the fish u catch but i feel like thats already a thing, i could try that u create stuff but what to create? and i need a way to earn points, ill credit whoever comes up whit ideas for their ideas if they want