- aggregating listings from across other sources / syndicated auctions is a cool idea. You probably still want a category focus. In the 90s, at one point the other dozen auction sites shared/exchanged their listings into one combined, shared set of listings, and eBay still killed them. But if you can choose a category, you can beat eBay.

- bit coin or other payment community, cool direction

- mixing in results from eBay is a great idea. Airbnb used craigslist listings to gain critical mass. Again, should work best within something narrow not broad

Good luck, you're going in the right direction