Are you looking to sell the bad debt outright, rather than engaging a collection agency? If so, there is a thriving consumer debt-buying industry, but finding a reputable company to purchase B2B debt is a bit more difficult. You will sell the uncollectable invoices for a fraction of their face value, and may have to have a minimum debt portfolio to qualify. Some basic research indicates that debt buyers appear to prefer invoices that have not yet been assigned to a third-party collection agency and will pay a higher price for those, but it's still pennies on the dollar.
If you are looking for a collection agency (and want to retain ownership of the debt, which should give you a larger portion of any monies collected), the Accounts Receivable Network has a number of resources about how to select one. If you've determined you simply want to sell the debt outright, however, debtbuyers.com is a good place to start your search, although the site also has listings of collection agencies and attorneys.
Regardless of the route you go, be sure to verify the business you select is reputable and law-abiding. There are many regulations about what they can - and can't do - and if the debt is non-U.S.-based, that further complicates things. Even if you no longer own the debt in question, sketchy practices can, at the least, affect your company's reputation.