You can convert 6,000 Marriott points into 3,000 Flipkart SuperCoins, which can give you benefits worth ₹3,000 during upcoming sales. However, keep in mind that you cannot use the entire amount on a single product. Flipkart has set category-wise limits for how many SuperCoins can be applied. Only on a few selected products are you allowed to use the full amount of your SuperCoins at once.
That said, I wouldn’t recommend converting your Marriott Points to SuperCoins at half the value. A better option would be to save up around 15,000 Marriott Points and then redeem them for a free hotel night. In most cases, that will give you greater overall value.