Learn Italian: Il proprietario stesso ha venduto la medesima macchina.

This highlights the core difference between emphasis and identity! When placed after the noun, as in il proprietario stesso, it acts as an intensive pronoun meaning "himself" (or herself/itself). When placed before the noun, as in la medesima macchina, it means "the same" indicating exact identity.

Could I say lo stesso proprietario instead of il proprietario stesso?

You could, but it completely changes the meaning. By placing the adjective before the noun, it changes from emphasizing the subject to showing identity. Lo stesso proprietario would mean "the same owner" rather than "the owner himself".

Is there any difference between using medesima and stessa for "the same car"?

In terms of meaning, no. Medesimo and stesso are synonymous when placed before a noun to mean "the same". However, medesimo is slightly more formal and literary, which adds a nice bit of variety to your Italian vocabulary.

Why use the passato prossimo (ha venduto) rather than the imperfect (vendeva) for an action in the past?

Because selling this specific car was a single, completed event. If you used the imperfect (vendeva), it would imply a habitual or ongoing action, as if he used to sell that exact same car over and over again!