For Washington, there is a Business and Occupation (B&O) tax which is a tax on services in Washington. This is different from a sales tax and the use tax which is on items that you purchase and didn't pay WA sales taxes on.
Generally, if you perform a service you should pay some type of B&O tax. On most of those actions you will also collect from your clients and subsequently pay a sales tax. Finally, if you buy something and bring it into or use it in WA without paying a sales tax you owe use tax.
Buying something online and paying the sales tax on it satisfies the use tax aspect.
The link provided by ThomasM125 above provides great descriptions and definitions. You will notice that most services that you provide are subject to B&O, but some may be excluded from Sales tax.
Here is another helpful link.
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