Hypothetically speaking, if a garage buys a car for 10K then I would expect them to be looking for 10% net return.
So the asking price would require to be marked up accordingly to achieve this.
Taking into account all the overheads as mentioned above, expenses and taxes, income tax, corporation tax, and VAT etc.
Also, a discount to the non P/X buyer or an over allowance on the p/x value would mean that they would require a screen price of circa 13.5k
A 10% net return on their investment is not unreasonable in my book.
If I sold a car to a dealer for 10k and a week later he had it up for sale at 13.5k on his forecourt, I would not be miffed as I know how the car sales system works.