"...the contrast isn’t between the skeptical Bereans, who insisted on Scriptural proof of what Paul was saying, and the credulous Thessalonians, who accepted it without question. Instead, the contrast is between the open-minded Bereans, who were willing and eager to examine the Scriptures and see if what Paul was saying was true, versus the hostile Thessalonians, who started a riot and got Paul in trouble with the authorities, even though he had proved from the Scriptures that Jesus is the Christ."
Acts 17:11-12 does indeed support Sola Scriptura in that the Bereans had tested the validity of the Apostle Paul's message by comparing it to the Old Testament Scriptures. If this method of discernment is not allowable, then it would make no sense at all for Luke to give these people a good reputation by calling them noble. Contrasting the response of certain people from Thessalonica does not change the argument. In fact, the context records Paul himself as appealing to those same Scriptures as the final court of authority in debating Jews (Acts 17:1-3).
"There is also another reason why this passage isn’t a good proof text for sola scriptura, which is this: The Christian faith contains doctrines that aren’t found in the Old Testament. What’s why even those who favor doing theology “by Scripture alone” don’t favor doing it “by the Old Testament alone.” While the Old Testament does contain prophecies that point forward to Jesus as the Messiah, the Christ, it doesn’t contain the whole of the Christian faith."
Saying that Paul and Silas did not have a compiled New Testament in their hands is nothing but a red herring. The fact that Jesus Christ was proven from the Old Testament to be the promised Jewish Messiah does not refute Sola Scriptura. The original intent of an author does not rule out a present application to broader conditions. It is therefore not out of bounds to cite Acts 17:11-12 as a supporting text for Sola Scriptura. The question regarding the extent of the canon, while related, is a separate issue.
The Bereans had used the Old Testament Scriptures to discern the message delivered by Paul and Silas. They had a love for God and His Word in their hearts. The Scriptures were read and searched out by these people in humility and eagerness. However, in Roman Catholicism it is maintained that scriptural proof is not necessary in order for a dogma to be true. The "laypeople" are not allowed to interpret Scripture for themselves:
"The task of interpreting the Word of God authentically has been entrusted solely to the Magisterium of the Church, that is, to the Pope and to the bishops in communion with him." (CCC # 100)
This kind of thinking is not in line with what we see taking place during the encounter with the Bereans and them accepting the gospel message. Paul did not direct these people to an infallible teaching office in order for them to test the validity of his message. The Old Testament Scriptures were sufficient for the purposes of Paul as he witnessed to the people as they verified the message that he delivered.