What is the smoothest and least expensive way to issue ACH payments for an online marketplace?

Well any provider that is going to provide you with a platform to conduct ACH payments, will require that the Sender is verified. Reason for this is the potential abuse for reversals. Unless of course you are will to have some deposit with the provider that will cover the reversals.

You might want to check out Bancbox (they seem decent) - See www.bancbox.com and you might want to check with Bill Wilson who is VP Product for Bancbox.

I haven't tried Bancbox, but it seems like a great service that is fully compliant.