What happens if a seller's non-compete is broken but I can't prove I actually lost any sales from it?

A buyer doesn't necessarily need to prove an exact dollar amount of lost sales just to establish that a non-compete was breached — breach and damages are somewhat separate questions in a claim like this. But proving the breach actually caused meaningful, compensable harm is generally necessary to recover more than a token amount, and courts can be skeptical of loss claims that are too speculative to actually measure.

Depending on how the non-compete clause itself was drafted, some purchase agreements include a pre-agreed damages figure for a breach, or make injunctive relief (an order actually stopping the competing activity) the primary intended remedy, specifically because proving lost sales after the fact is genuinely difficult in most real businesses. It's worth checking whether your clause offers one of these easier paths rather than assuming you're stuck proving a precise revenue loss figure from scratch — the clause's own wording may already anticipate exactly this problem.