Doing a lot of work to a car before you trade or sell it to a dealer is not generally a good idea.  In most cases, you won't recapture the cost of the repairs.  They can do the repairs for a less than you can.  Not to mention, you are selling to a wholesaler who needs to sell it again to the end user and still make a profit.

A home sale is totally different.  The owner is selling the home to an end user.  Since the buyer, in many cases, is using their available funds for the down payment and purchase costs, they don't have money to spend on repairs or decorating the home.  They would need to live in it "as is" for a while which may not be as appealing as finding a home that is refurbished, up-to-date, and ready to move into.

Even if the buyer would be willing to get a home improvement loan after the sale, it would be a separate loan at a higher interest rate making their payment higher than financing it all in one mortgage at the lower first mortgage rates.