Buying a pre owned car reccs?

Hi so I have a little Nissan car with a little miles and I even got the radio and wheels and tint done haha but I got told by the mechanic that I need a new one due to some issues sometime soon.

Do u guys have any recommendations on how to navigate these situations ?

I don’t really have parents or anyone to rely on lol

Like idk who gives me the best deal for a trade in for me to get another car or even trade in at one place and buy at another I’m just exhausted that this happened after all the upgrades and new tires that I got 😭

I’ve heard of carmax and caravana but i know that apparently caravana isn’t good or something ?