I have an outstanding debt from a client based in Seattle.

What kind of debt is it? Do you have something in writing to show that the person owes you money? I don't recommend collection agencies. Many attorneys, like myself, provide free consultations to see if we can help.

Does the client have any assets in Hong Kong? If not, why would you approach a collection agency there, especially if they are not licensed and able to do business in Washington State?

I agree with the other attorneys that it depends on a number of factors including the type of debt, any applicable agreements, where the client has their assets, and what form of security interest you may have in such assets. In the U.S., you may be required to go through a legal action to recover assets from your client, yet further information is needed. Working with a qualified Washington state attorney can assist you in evaluating your claims, assuming your arrangements with the client is subject to U.S. law. Good luck!