If your requirements are such that the employees need to invoice you first before they get paid, I don't know how you can get around the seller account requirement.
However, if all you're trying to do is simply pay your employees, and manage their invoices through a separate system, something like PayPal could work -- I think really any payment gateway could work if you could push your requirements to the point that you simply pay your employees (one process) and handle client invoices through a second process.
Last time I checked, Paypal lets you transfer to family and friends without their percentage charge, which you could possibly bend to meet your employee payment requirement, if you can offload the invoicing piece to a separate system.