Can I claim against a seller's estate if they die shortly after the deal closes?

Generally yes — a valid claim against a seller, whether for an indemnity obligation, an outstanding vendor take-back debt, or a judgment, doesn't disappear simply because the seller has since died. It becomes a claim against the seller's estate instead, to be dealt with by the executor as part of administering the estate alongside its other debts and obligations.

There's a practical wrinkle worth acting on quickly, though: there's typically a process and timing expectation for creditors to bring claims against an estate, and an estate can be distributed to beneficiaries in ways that make recovery considerably harder if a claim isn't raised within a reasonable time. A buyer with any live or developing claim against a seller who has died should get advice promptly about how to protect that claim properly within the estate administration process, rather than assuming it can simply be raised whenever convenient — waiting risks the estate's assets being distributed before your claim is formally in the picture.