Does losing one big customer right before listing tank my price?

It can meaningfully affect price, though "tank" is usually too strong unless that customer represented an outsized share of your revenue. Losing a major customer right before listing does two things at once: it reduces your near-term earnings, which affects the numbers a valuation is based on, and it highlights customer concentration as a live risk rather than a theoretical one, which can make buyers more cautious about relying on your remaining customer base generally.

The nuance is that how you explain the loss matters as much as the loss itself. A customer lost to circumstances outside your control, with a credible plan already in motion to replace that revenue, tells a different story than a loss that suggests a broader weakening in your customer relationships or competitive position. Buyers and their advisors will want to understand which one they're looking at.

If this has just happened, it's worth being upfront about it rather than hoping it goes unnoticed during due diligence, since financial statements and customer contracts will surface it regardless. A business lawyer and accountant can help you frame the loss honestly and show what you're doing to address the underlying concentration risk going forward.