If you have comprehensive insurance, the insurance may pay for a replacement windshield.
As far as the rear seat is concerned, you might visit an upholstery and trim shop and see what the charge would be to do what is called an “insert” of a new section of upholstery on the seat. I had the seat repaired years ago on my 1978 Oldmobile Cutlass 4-4-2 when the vinyl upholstery wore through and the price was quite reasonable.
I have found one of the best sources of getting information about putting a car into shape for resale is a used car manager at a new car dealer. Most of these managers are quite friendly. They know the good trim shops, detail shops, transmission shops, etc. On trade-ins they choose to resell, they often farm the work out to these shops. A new car dealer has several departments: new car sales, used car sales, service, parts, and sometimes a body shop. Each department is expected to show a profit. Often, the used car manager finds it is less expensive to have the work done by an independent shop than to have the work done in-house.
At any rate, check into your insurance for the glass, see an upholstery shop about the seat, and spend half a day cleaning up the car. First impressions count and you may recoop the money spent on these items on a trade-in. After all, there are two separate transactions when you trade cars–you are buying a car from the dealer and he is buying a car from you. Even if your car is going to the auction, it will fetch the dealer more if it gives a good impression.