Can I claim against a seller who moved to another province after the sale closed?

Yes — a seller relocating to another province doesn't defeat a valid claim connected to an Ontario business sale. Ontario courts can generally still take jurisdiction over a claim arising from the sale, and once you have an Ontario judgment, Canada has mechanisms that generally allow it to be recognized and enforced in other provinces, so a move alone doesn't put the seller permanently out of reach.

What the move genuinely adds is practical complexity rather than a legal barrier. Serving the seller with legal documents, and later enforcing and collecting on a judgment once obtained, both take extra steps when the person is out of province — locating current assets, following the right enforcement procedure in that other province, and coordinating between counsel in two jurisdictions if it comes to that. None of this makes the claim impossible, but it does make early legal advice more valuable than usual, since planning the right approach from the outset saves real time and cost later.