Should I be concerned if a business has no written contracts with its biggest customers?

Reasonably, yes. Relationships built on trust and history rather than signed agreements are common in many Ontario small businesses, and they aren't illegal or automatically a sign of poor management, but they carry a real risk for a buyer: an unwritten arrangement can be far more easily ended, renegotiated, or simply not renewed by a customer once ownership changes hands, since there's nothing legally binding the relationship to specific terms, pricing, or duration in the first place.

This matters more the more concentrated your revenue is in a small number of customers. A business with several large customers on informal handshake arrangements is more fragile than its financial statements suggest, because those customers have no contractual obligation to keep buying from you after the sale, whatever their history with the previous owner.

Confirm which customer relationships are contractual and which are informal, and treat customer concentration on informal terms as a real valuation and risk factor, not just a technical gap to note and move past. Consider whether formalizing key relationships before or shortly after closing is realistic, and price the deal to reflect the actual risk. A Treadstone business lawyer can help you think through how this affects deal structure.