Are your parents are going to live with you long term? I would suggest getting a Pod initially. Just focus on decluttering and cleaning each room thoroughly. Prepare a room for your parents void of decorations and clutter, which would allow them to make it their own. I would put all of their non-essential possessions in the Pod. Then start sorting through the Pod. Put items they want to keep in the house. Label items to discard, donate, or sell. You may also decide that some of your parents items are nicer than yours and swap them out. List items on ebay or craiglist. Whatever doesn't sell there do a yard sale. Donate the items that doesn't sell.