Learn Arabic (Modern Standard): ذلك البائع مشغول الآن.

The word ذلك means "that" and is used to point out something further away. If the seller was standing right next to us, we would use هذا (this). Because بائع (seller) is a masculine word, we use the masculine "that": ذلك.

Why does البائع (seller) have ال (the) attached to it? Doesn't that make it "that the seller"?

In Arabic, to group a demonstrative and a noun together into a phrase like "that seller", the noun must have ال (the). If you leave ال off and just write ذلك بائع, it breaks the phrase apart and creates a full sentence meaning "That is a seller."

Where is the word for "is"?

Arabic doesn't use words for "is" or "are" in present-tense sentences. You simply put the subject, ذلك البائع (that seller), next to the description, مشغول (busy). Because مشغول is indefinite (it doesn't have ال), it completes the sentence naturally: "That seller [is] busy."

I notice ذلك is pronounced with a long "a" sound (thaa-li-ka), but I don't see a regular alif (ا). Why is that?

ذلك is one of a small handful of common words in Arabic (just like هذا and الله) where the long "a" sound is pronounced after the first letter but is not written with a standard alif. It is an exception you just have to memorize!