Offering credit, or "trade financing" to your customers does not require any special documentation. Most vendors (hardware stores, trade magazines, lawyers, etc.) offer their regular customers 30 days to pay their bills. Some offer up to 90 days. The key is making sure that your customers will pay you reliably and on time. The best way to do this is to ask them to supply you with three "trade references"--other vendors who have given them credit in the past--to demonstrate that they have a track record of paying their bills. It's also important to set a credit limit for each customer to make sure that you don't get stuck holding the bag.