car boot sale vs sales event

car boot sale and sales event both are nouns.

Word	Noun	Adjective	Verb	Adverb
car boot sale	Yes	No	No	No
sales event	Yes	No	No	No

As nouns, sales event is a hypernym of car boot sale; that is, sales event is a word with a broader meaning than car boot sale:

car boot sale: an outdoor sale at which people sell things from the trunk of their car
sales event: an occasion (usually brief) for buying at specially reduced prices

Other hypernyms of car boot sale include cut-rate sale, sale.

car boot sale (noun)	sales event (noun)
an outdoor sale at which people sell things from the trunk of their car	an occasion (usually brief) for buying at specially reduced prices