Learn Russian: Говорят, хорошие билеты быстро раскупают.

No, хорошие билеты is the direct object in the accusative case (which looks exactly like the nominative plural because tickets are inanimate). The sentence literally translates to "[People] buy up good tickets quickly." Using a third-person plural verb without a subject pronoun is a common way in Russian to express passive English ideas like "are sold out."

Why use a verb based on "buying" (раскупают) when the English translation says "sell out"?

English focuses on the items "selling out," but Russian often focuses on the consumers "buying them all up." The prefix рас- indicates exhausting a supply or distributing an action thoroughly. While you could use a reflexive verb like распродаются (are sold out), раскупают is extremely natural and emphasizes the high demand from buyers.

Why is there no subject like они before говорят?

Using a third-person plural verb with no subject pronoun is the standard Russian structure for "they say," "people say," or "rumor has it" (an indefinite-personal sentence). If you added они (они говорят), it would mean a specific group of people you've already mentioned are saying it, rather than a general statement.

Why is the imperfective verb раскупают used here instead of the perfective?