What happens if the seller refuses to help transition customer relationships as promised?

This depends on whether transition assistance was an actual contractual obligation — a specific covenant in the purchase agreement or a separate transition or consulting agreement — rather than something merely expected informally based on how the deal was discussed. If it was a genuine, documented covenant, the seller's refusal to provide it is a breach, and you generally have a damages claim for the losses that refusal causes.

Courts are generally reluctant, though, to force someone to personally perform an ongoing services obligation like helping transition customer relationships — that kind of order is unusual for personal-services-style commitments, so compensation for the losses the refusal caused is more likely than actually compelling the seller to show up and help. This is exactly why transition obligations that genuinely matter to a buyer are worth pinning down with specific, enforceable terms — and sometimes a financial incentive tied to performance — at the time of the deal, rather than left as a vague handshake-level expectation that turns out to have no real teeth.