Before you agree to sell to any customer on credit, you should check their creditworthiness. Online credit ratings are inexpensive. You can also check trade references, ideally following up with a phone call in case the supplier was unwilling to put any concerns in writing.
If you have any doubts about their creditworthiness, you should ask yourself whether you want to offer them credit at all. You may be able to negotiate cash in advance, or might be better off not having them as a customer. In any case, you should only allow customers a sensible credit limit, reflecting how risky you think they are and how much your business could afford to have tied up in overdue payments or bad debts.
Make sure you have a good credit control system that encourages prompt payment. This should include everything from having the right terms and conditions to chasing up payments. Watch out for any warning signs such as increasing payment delays.
You may also want to consider using a factoring company. In return for a fee, the factor advances you most of the value of outstanding invoices, chases payment on your behalf and takes on the risk of bad debts.