Why capitalize car here? Why capitalize your? *shakes head* A comma is not end punctuation; the first word after a comma is not the first word of a sentence.
Also note where commas have been added and where they have been removed.
You buy a large antique chest at a car boot sale. When you arrive home, you store some things in it. The next time you open the box, your items are gone, and in their place is a note that reads, “Very intriguing. Please send something else!”