I think a web3-enabled eBay alternative on Hive is a great idea. This will be another real use case, onboarding and retention project for Hive like Distriator. You should try and get funding from the Decentralised Hive Fund (DHF), failing that you could try and do crowdfunding from future Hive users like the Holozing project.
RE: True Web 3.0: The Crypto Economy