Build an integrated market, initially aimed at competing with Craigslist, and maybe expanded to target other markets later (Ebay, etsy, etc). Craigslist first because its similarly simple UI and design is the easiest to recreate within Reddit.

There's tons of subreddits for connecting buyers and sellers of niche goods, but the actual buying and selling has to happen elsewhere. It's stupid that Reddit hasn't capitalized on that yet.