Depending on who you are buying from, they may be a tax-exempt dealer. For example, if a dealer sells from a state that does not collect sales tax to a state that has exemptions for that category based on low volume or type of product or also does not collect sales tax. It seems that would be hard to find, so it's likely anyone you are buying from is just not paying the required taxes. If you receive goods from a dealer and they don't charge you sales tax/remit taxes to your state, and your state requires it, you should pay the use tax. Typically sales/use taxes are interchangeable. I'm not tax attorney, but the point is, someone has to pay the tax in most cases.
People also cross the Oregon/Washington border to purchase goods sales tax free to take back to Washington.