Learn Russian: Хороший хлопок всегда быстро продаётся.

While -ся often makes a verb reflexive (doing an action to oneself), it is also used to create a passive voice. When attached to an imperfective verb like продавать (to sell), it shows that the subject (хлопок) is receiving the action, not doing it. Since cotton can't literally sell itself, it translates to "is sold".

Could we use a perfective verb with -ся here to say it was successfully sold?

No, the passive -ся structure is used with imperfective verbs to describe ongoing processes or general facts. If you wanted to use a perfective passive for a completed result (like "the cotton has been sold"), you would use a short-form participle, such as хлопок продан. Also, the word всегда (always) means this is a repeating action, which requires the imperfective aspect anyway.

Does хлопок have two meanings? I remember seeing it translated as "a clap".

Yes, they are spelled the same but pronounced differently depending on the stress! хло́пок (stress on the first syllable) means "cotton". If the stress is on the second syllable (хлопо́к), it means "a clap" or "a pop". Context makes it clear here, as you wouldn't typically buy or sell a clap.

Why are the adverbs всегда and быстро placed right before the verb?