How can I find a trustworthy car seller in San Diego?

I live in San Diego and I'm looking for a decent looking car for cheap (like 5k or less) but am new to OfferUp and have ZERO idea on how to find a trustworthy seller. From what I've heard, a lot of people try to sell "rebuilt/salvage title death traps" on places like Facebook Marketplace and OfferUp and I have no clue where to go. Any help?