Is there any historical record to prove when this occurred? Or, is this yet to happen?
As the context of Joel 3 suggests it is set in the future.
The book of the prophet Joel is notoriously difficult to date; it is not even known (for sure) whether this was a pre-exilic writing or a post exilic writing. However, Joel 3:1 suggests that it was written before or during the Babylonian exile. So let us assume that it was sometime in the period 700 BC - 500 BC.
With this in mind, Joel 3 admits of two basic interpretations:
1. Greek Slave Traders
This is one of the most popular interpretations of many such as:
(6) Unto the Grecians.—Javan, the Hebrew name for Greece, is mentioned in Ezekiel as one of the representatives of the heathen nations who “traded (with Tyre) the persons of men and vessels in her market” (Ezekiel 27:13). The Grecian traffic in slaves was enormous.
Joel 3:6. The children also of Judah, &c., have ye sold unto the Grecians — The descendants of Javan, Genesis 10:2; Genesis 10:5. They trafficked with Tyre, and traded in slaves, Ezekiel 27:13. It was customary for the merchants of the neighbouring countries, particularly of Tyre and Sidon, to buy the children of Israel for slaves of their conquerors, in order to sell them again: see 1Ma 3:41.
the Grecians Heb. Yewânîm, elsewhere in the sing. (collectively) Yâwân (Genesis 10:2; Genesis 10:4; Ezekiel 27:13; Ezekiel 27:19 (?); Isaiah 66:19; Zechariah 9:13; Daniel 8:21; Daniel 10:20; Daniel 11:2); i.e. the Ionians (Ἰάϝονες, in Homer constantly), the name by which the Greeks were regularly known among both the Hebrews, and other Eastern nations, on account, viz., of the fact that the Ionians (whose colonies extended over a large part of the W. coast of Asia Minor, and many of the Aegean islands) were most active commercially in ancient times, and hence were best known to other nations.
2. Greek Rulership
It is well-known that the Greeks conquered Jerusalem and Judah late in the 4th century BC. When Alexander died, Judah became the conquered possession of the Seleucid kingdom with occasional interruptions by Egypt.
The verse in question:
Joel 3:6 (NASB)
and sold the sons of Judah and Jerusalem to the [a]Greeks in order to remove them far from their territory,
a. Joel 3:6 Lit sons of Javan
The NASB footnote says "Greeks" is literally "Sons of Javan". He was the fourth son of Japheth.
Genesis 10:2 (NASB)
The sons of Japheth were Gomer and Magog and Madai and Javan and Tubal and Meshech and Tiras.
"Sons of Javan" = Greece and Greek people
Also serving as the Hebrew name for Greece or Greeks in general
This question is largely about an uncertain historical context. If we can answer this question, we can more accurately date the Book of Joel. We must understand both the time and the meaning of the passage together.
We should always consider the nature of prophecy to have multiple fulfillments.
Dual Fulfillment (Wikipedia)
The dual fulfilment... or dual prophecy or duality in prophecy or present and future application is the mainly Christian idea that some prophecies in the Bible have both a short-term and long-term fulfilment.
Also, we must look at a few other verses in the passage to get the context of the passage itself.
Here are some other contextual points to observe:
Joel 3:4 (NASB)
Moreover, what are you to Me, O Tyre, Sidon and all the regions of Philistia?...
God is addressing neighboring adversaries of Judah.
the act or an instance of selling out
But, we shouldn't get too much hermeneutical insight from English dictionaries alone. Consider that Jesus was "sold-out" by Judas.
Thirty pieces of silver (Wikipedia)
The phrase is used in literature and common speech to refer to people "selling out", compromising a trust, friendship, or loyalty for personal gain.
Hence, we should see this as a reference to Jesus as the sold-out Christ. The collective Bible authors are saying in Joel that Jerusalem/Judah will be "sold-out" just as Jesus was "sold-out". With the verses after v6, that Jerusalem will make a comeback, this bolsters Jerusalem being a type of Christ since Jesus also came back from the grave.
Typology (theology) (Wikipedia)
Typology in Christian theology and Biblical exegesis is a doctrine or theory concerning the relationship of the Old Testament to the New Testament... For example, Jonah may be seen as the type of Christ in that he emerged from the fish's belly and thus appeared to rise from death.
This sale is described in the past, but that could suggest a past action that weakened Jerusalem, while Greece had not yet taken Jerusalem. It could be taken either way, so we must remain open minded.
There is debate about when Joel was written. (This validates the original Question, that it is indeed worth researching and asking.)
Book of Joel (Wikipedia)
As there are no explicit references in the book to datable persons or events, scholars have assigned a wide range of dates to the book.
The mere mention of the Greco empire, not Persia as in the Books of Daniel and Esther, bolsters the idea that Joel was a contemporary of Greco dominance. This dates the Book of Joel itself closer to Malachi than Ester. That makes a reasonable likelihood that it could have already happened in the past.
"Greek" can also mean "Gentile" in the New Testament. Consider Paul's equation of "Greek" to "non-Jew" in his juxtaposition to Galatia...
Galatians 3:28 (NASB)
...neither Jew nor Greek...
Paul wrote that way because Jewish readers would understand it that way. This means the Jewish community in the New Testament likely interpreted Joel to suggest non-Jews in general, at least on some level or in greater fulfillments. So, the time of the "sale" you are looking for could be a sale to the non-Jews/nations at large, possibly even referencing Revelation, an event still yet to come...
Revelation 11:2 (NASB)
...for it has been given to the nations; and they will tread under foot the holy city for forty-two months.
Joel 3:7 (NASB)
behold, I am going to arouse them from the place where you have sold them...
This seems to have already happened with the restoration of Israel in 1948. That's not to say it couldn't happen even more, but it looks like 3:7 is being fulfilled in our own time. That helps to provide some context for v6.
Joel 3:8 (NASB)
Also I will sell your sons and your daughters into the hand of the sons of Judah, and they will sell them to the Sabeans, to a distant nation,” for the Lord has spoken.
The Sabaean people were the South Arabian people.
So, these "sons of Javan" eventually get sold to a potentially Arab people.
When did this happen? Already and yet to come.
In AD 332 for sure, when Greece sacked Jerusalem. But, it can't be the Siege of Jerusalem in 636 since those were Arabs, not descendants of Javan.
Joel puts this as an event in the past, not mentioning Persia as he likely would have if it had not yet happened. So, it seems to already have happened, at least using hermeneutics of "face value" meaning. Thus, even if it hadn't already happened, Joel wants us to think at if it has.
But, it is the multiple fulfillment meaning we must consider at the forefront, along with the Christological reference of being "sold", just as Christ was sold. This more strongly suggests the presence of multiple layers of meaning, especially Christological. That means this has an apocalyptic undertone; this is about the End Times described in Revelation.
Ultimately, it looks like it will happen once more when Revelation 11:2 is fulfilled.