It's possible to compete, but to quote Ben Horowitz, you need to be not just better, but many times better, to avoid the inertia of people using the status quo. To that end, I suggest you consider going the Etsy route and catering to a specialist market, building your product out, and then expanding. Taking on eBay in a frontal attack is doomed to failure.

I like the messaging idea - build that into your support side as well, possibly by crowdsourcing and rewarding loyal users. And I also like the ability to flag on craigslist which is seriously missing on EBay. User-driven tagging would be a boon; I trade synthesizers/audio gear fairly frequently on EBay and nothing is more irritating that searching for a synth and getting pages of accessories that mention that device in the headline or text. I loathe and despise seller spam.

Tech stack, I have no ideas.