Miles must be used immediately for a specific redemption; you cannot buy them to hold in your account for future use.
Platforms like Citi PayAll or CardUp allow you to pay rent, taxes, or bills with a credit card for a small fee, effectively "buying" miles for roughly 1 to 1.5 cents each.
While third-party "mileage brokers" or marketplaces like Carousell may offer lower prices, using them is by Singapore Airlines.