Does the seller have to sign a statutory declaration on closing day, and what's actually in it?

Yes, sellers are routinely asked to sign one or more statutory declarations at closing. Unlike an ordinary signed statement, a statutory declaration is sworn before a commissioner or notary and carries real legal weight if it later turns out to be false — which is exactly why buyers ask for it on points they can't independently verify.

Common content includes confirmation that there are no unpaid employee wages, unremitted source deductions, or outstanding HST amounts that could otherwise expose the buyer to liability; confirmation of the seller's residency status for tax purposes; and confirmation that there are no undisclosed liens, legal proceedings, or other claims against the business beyond what's already been disclosed. It sits alongside, but is distinct from, the ordinary representations already made in the purchase agreement, giving the buyer an added layer of personal accountability on specific factual points. If you're a seller asked to sign one, read it as carefully as the purchase agreement itself, since a false statement here carries its own consequences.