Can a seller take security over just my accounts receivable instead of the whole business?

Yes. Vendor take-back security does not have to cover the entire purchased business, and a seller can register a security interest under Ontario's Personal Property Security Act limited to a specific, defined class of collateral, such as accounts receivable, rather than taking a blanket general security interest reaching every asset the business owns.

A narrower security package like this can be easier for a buyer to agree to during negotiation, since it leaves other business assets, such as equipment or inventory, unencumbered and available to secure other financing the buyer may need, whether from a bank or another lender. At the same time, it gives the seller meaningfully less protection than a full general security agreement would, since on a default the seller's recourse is limited strictly to whatever falls within the defined scope of the security actually granted, in this case receivables specifically. How much security a seller is able to obtain, and how narrowly or broadly it is defined, ultimately comes down to negotiating leverage between the parties, not any standard practice.