Some sellers will have an inspection done in advance of putting their home on the market, but this is not always the case. Unless you receive written documentation from the Seller's Representative that an inspection (structural and pest) has been conducted and any recommended repairs have already been completed, protect your prospective investment! Write your offer contingent upon an inspection and acceptance of the subsequent report that is provided.
It is your responsibility to select an inspector. You will want someone who is licensed by the State of Washington and who carries liability insurance. This means that if the inspector misses something costly to repair/replace, said cost will be borne by the inspector who goofed. Now is not the time to ask for help from a relative who does this sort of thing in his spare time.
The cost of most inspections varies and is usually related to the size of the home and where it is located. The least expensive fee is about $300; some firms charge by the hour. Feel free to ask about the inspection fee and how it is determined when checking out different firms.
If you are unfamiliar with who to call, ask me. I maintain a list several good local home inspectors.