If you the owner (on title) of the house, you may remove any personal property left by anyone in the house or in the yard at any time and charge the previous owner for removal, unless you are a custodian of that property (meaning, you were his landlord before foreclosure for example). In that case, RCW 63.29 would apply and you would have to report to as unclaimed to the Washington Department of Revenue before you can dispose of it.
DISCLAIMER: This is not legal advice because you and I do not have an attorney-client relationship.
I agree with the previous answer. You could give a short note explaining that you are not a storage facility and that her permission to enter the property will end on a date certain and she should get the stuff she wants out before that or it will be disposed of without further notice. You actually did purchase the personal property that was listed in the notice of trustee's sale. If this becomes an issue and you cannot work with her you should contact law enforcement and a lawyer for a no-trespass order.
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