Buying an used car/suv in Houston: Better from big website or from local retailers? Pros and cons? Best deal for family van (mommyvan)

Hi, I guess the title is self explanatory. I'll be moving to Houston soon and will need a family car. plus maybe a smaller sedan for me.

I will be looking for used cars. I see there are some sponsored websites that are like big sellers. Is it worth to get a car there or better to look locally?

Thanks and best regards