i start my used car purchases with general maintenance, and what i call a "class 5" cleaning.
seats get uprooted and i shampoo everything. i use a small squirt of laundry detergent in a spray bottle filled with hot water, a stiff brush,...and ALL my garage towels.
scary what you find under the seat..nail clippings, old french fries..it is really gross. i want the fries and clippings to be mine