Exactly, you can buy selected invoices from suppliers of solid companies such as a Target. They'll pay. The risk is if you take on invoices from companies who are selling other smaller companies(risk of customer not paying invoice/going out of business). You can buy invoices from smaller companies because they aren't the credit risk, their customers are. Yes, check with your state on any regulation. There is tons of this going on since the banking system is locked up. Companies are paying higher and higher rates for operating capital and more and more will be factoring which is already the mode of biz in Europe and Latin America and this will be a larger and larger component to US biz finance. Long gone are the days of low interest lines of credit from your local bank.
You advance the supplier less than what your cut will be when you buy the invoice, when the customer pays you the full amount of the invoice to your lock box, you advance the balance less your cut to the supplier. I deal with some that are family so don't have a lock box with them, but you must control the invoice payment with a lockbox with any other company.
If you don't want to deal with details, call a few factoring companies and ask if they have investor programs. They might offer 1% a month for additional capital for hands off investors.