Can I sue a business broker if they knew the seller was misrepresenting the numbers?

Potentially yes, and this is a separate claim from any indemnity claim you have against the seller under the purchase agreement — pursuing one doesn't require giving up the other. A broker who knowingly participated in passing along, or actively helped present, false information to a buyer can face independent liability in negligence, misrepresentation, or breach of a professional duty, depending on the broker's actual role in the transaction and any regulatory obligations that applied to them.

Each claim has its own separate elements to prove, though. The seller's liability generally flows from the representations made in the purchase agreement itself; the broker's liability depends on what the broker actually knew and did independently of the seller, and the broker's likely defence — that they simply passed along what the seller told them without reason to doubt it — is a genuinely different fight than anything you'd argue against the seller. Both claims are worth assessing together with a lawyer rather than assuming only one target is available.