OSU Extension's publication on "Is Starting a Farm Stand Right for You" contains a section on land-use permits:

Land-use permits. When working with your county to establish a farm stand, the most critical variable to research is your property’s zoning. Farm stands are an acceptable use on land zoned as Exclusive Farm Use, Rural Service and Rural Residential. Zoning will determine the permitting process, as specific rules pertain to each zone. Zoning also affects the cost of the required permits.

Permits on EFU properties allow for the sale of incidental retail items in the farm stand, such as bottled water or shopping bags with your farm logo. They also allow the use of fee-based activities to promote the farm stand, such as farm tours, food-preparation classes and outdoor farm-to-table dinners. However, income from these retail items and activities must not make up more than 25% of the farm stand’s total annual sales.

A permit for a farm stand consisting of off-street parking and signs, plus a card table and honor system of receiving payment may be less expensive than a farm stand that has a building structure. Permit fees depend on the county where the farm stand is located.

I would suggest reaching out Multnomah County planning office for information on required permits for a farm stand on your property. They have a fillable "Ask a Planner" online form to complete.