Ask HN: What's the best service for sending money to your users?

If a start-up wants to support either paying its users or facilitating payments between users (possibly keeping a percentage of the payment), is there an alternative to PayPal?

Are there gotchas associated with this process? For example, ebay has buyers send payment directly to sellers and then bills the seller for ebay's fees after the fact. Is there a reason for this approach? Are there other online businesses that facilitate transactions, and what payment services/approaches do they use?