Vowels and dipthongs in English can look similar, and have many different pronunciations. However, from your description it seems that the speaker was changing their pronunciation of the article "a", to emphasise or enunciate parts of the phrase, and this is very common.
In my own British English accent, I would probably say;
"I want to buy a /ə/ car" - flat tone, no emphasis. It isn't important to me now.
"I want to buy a /eɪ/ car" - raised tone, stronger emphasis. It is important to me now.
In practical terms, both are the same and you normally shouldn't worry that they mean anything different.