What happens if key customers all leave right after I take over and the seller knew they were unhappy?

This depends on whether the purchase agreement contained a representation touching on customer relationships — commonly something like no known disputes or dissatisfaction involving key customers — that turns out to have been untrue when made. If the seller knew important customers were unhappy and didn't disclose it, and that representation existed, the resulting departures after closing can support an indemnity or misrepresentation claim once the loss becomes apparent.

If the agreement made no such representation about customer relationships at all, customer attrition after a sale is generally treated as ordinary business risk that the buyer assumed by purchasing the business — not every disappointing development after closing traces back to something the seller is legally responsible for, even if the seller privately suspected it was coming. The difference between these two outcomes really comes down to the exact wording of what was represented, so reviewing that specific clause, and what the seller actually knew and when, is the essential first step before assuming either that you have a strong claim or none at all.