How much are you willing to spend per transaction? With Stripe or Braintree it’s trivial but expensive.
Are you targeting just the US market and dollars? Going directly with a bank is cheaper but more complex [1]. And unless you’re adding significant value you’ll probably need a money transmitter license in your state.
And if you’re planning on letting any asshole on the internet use this, you need an anti-fraud strategy because settlement times are measured in days and there are plenty of ways that money can be sent but never refunded.
Good luck!
[1] http://engineering.gusto.com/how-ach-works-a-developer-persp...
