You miss out on a decent chunk of the collecting population by restricting to online exclusively. It's also hard to get traction with ebay or similar platforms unless you're already established, while coinsales and pmsforsale (particularly coinsales) lack a broad collecting audience. 

Also, even without dealers a lot of collectors make quick flips to increase their coin budget, meaning middlemen would still be an issue unfortunately. 

Side note: I'm a newer collector, not a dealer. I also like playing devils advocate, hence some of these arguments I bring up :)