God never gave the Pharisees or the Church the authority to add or take away from Scripture:
The Pharisees confronted Jesus by asking: "Why do Your disciples not walk according to the tradition of the elders, but eat their bread with impure hands?" (Mark 7:5). This was in accord with the oral tradition not according to the Law as handed down by God. The Lord accused the Pharisees of "teaching as doctrine the precepts of men" (Mark 7:7). The Scriptures said absolutely nothing about washing one’s hands before eating. Nonetheless, the Pharisees enforced ceremonial handwashing as if God Himself had ordained it. In this way, they had elevated the teachings of men to the same level of authority as God’s inspired Scriptures. Paul added: "Therefore do not let anyone judge you by what you eat or drink, or with regard to a religious festival, a New Moon celebration or a Sabbath day. These are a shadow of the things that were to come; the reality, however is found in Christ." He goes on to say: "Since you died with Christ to the basic principles of this world, why, as though you still belonged to it, do you submit to its rules: Do not handle! Do not taste! Do not touch!. These are all destined to perish with use, because they are based on human commands and teachings. Such regulations indeed have an appearance of wisdom, with their self-imposed worship, their false humility and their harsh treatment of the body, but they lack any value in restraining sensual indulgence."
Jesus also emphasizes: "Nothing outside a man can make him 'unclean' by going into him. Rather, it is what comes out of a man that makes him 'unclean.' After he had left the crowd and entered the house, his disciples asked him about this parable. "Are you so dull?" he asked. "Don't you see that nothing that enters a man from the outside can make him 'unclean'? For it doesn't go into his heart but into his stomach, and then out of his body." (In saying this, Jesus declared all foods "clean.") He went on: "What comes out of a man is what makes him 'unclean.' " Jesus is continuing His assault on the Pharasaic Laws.
Jesus also warns: "I tell you the truth, until heaven and earth disappear, not the smallest letter, not the least stroke of a pen, will by any means disappear from the Law until everything is accomplished." The Law here in not the human law of the Pharisees or the Tradition of the RC Church - it is the Holy Scripture.
You reject the New Testament then?
SD
"The Law here in not the human law of the Pharisees or the Tradition of the RC Church - it is the Holy Scripture."
More specifically, it is the Septuagint Old Testament, and not the New Testament at all, given that it had not been written yet when Jesus said that. Jesus' biblical quotes almost invariably use the Septuagint form (where it varies from the Hebrew text of the Massoretes).
That the NEW Testament is "Scripture", and contained under what Jesus said there, is a tradition of the Catholic Church, dating from the 3rd or 4th Century. What Jesus is talking about here was the Jewish Scripture, not the New Testament.
"So you must obey them and do everything they tell you."
There's no limitation to Scripture in this statement.
And which set of Scriptures are you referring to? Various Jewish groups relied on various canons of Scripture.
The Canon of the Old Testament
And what was "the seat of Moses"?
Refutation of James White: Moses' Seat, the Bible, and Tradition, Part V