Paypal/eBay - couldn't find any published figures on this, but looking at eBay's revenues vs. PayPal's revenues, it'd appear that it's a substantial but not dominant share of their revenue.

Pricing model - no, it's great, but as you grow, it becomes uneconomical.

What's stopping them? Fraud. Fraud fraud fraud. They will end up with an approval process, as if they get hit big a few times (they will), their merchant acquirers/clearers will insist on it.