"We need to build this" is one of the most expensive sentences a founder says, and it's usually wrong — not because building is bad, but because the instinct fires for the wrong reasons: it feels cheaper than it is, it feels like control, and engineers like building.
Three questions, in order
Is it core to why customers choose you? If not, buy it. Auth, billing, email, search — someone has spent ten years and a hundred engineers making these better than you will. Building them is ego, not strategy.
If it is core — do you need it now, or do you just think you'll need it? Most "we'll need to scale this" is a problem you can buy your way out of for two more years, by which point you'll know far more. Wait is a real option, and often the right one.
If it's core and you need it now — is anyone going to own it? An unowned build is the worst outcome of all: you pay to build it, then pay again when it rots.
The trap is treating every build-vs-buy as a fresh debate. It isn't. Run it through the tree, and most of the time the answer is "buy it, and spend the saved months on the one thing only you can build."