Learn Italian: Nessuno vuole comprare quella macchinona perché costa troppo. - Nobody wants to buy that big car because it costs too much.

The suffix -one or -ona is an "augmentative", meaning it makes a noun larger. By dropping the final vowel of macchina (car) and adding -ona, you get macchinona (big car). It keeps its feminine gender here, which is why it is paired with the feminine word quella (that).

Doesn't Italian usually use double negatives? Why is there no non here?

You only need a double negative (using non) if the negative word comes after the verb. Since nessuno (nobody) is the subject and comes right at the start of the sentence before the verb vuole, the non is dropped.

Why doesn't troppo end in -a to match the feminine macchinona?

Because here, troppo is acting as an adverb modifying the verb costa (it costs too much). Adverbs never change their endings. You only change the ending of troppo when it is an adjective directly describing a noun (for example, troppe macchinone for too many big cars).

Why is comprare left in its infinitive (dictionary) form?