I'm going to take a guess at this based on what little I know of Canadian law.
I know for a contract to be legally binding, both sides have to receive something of value. This is why people sell things for one dollar all the time. If you keep the deposit, and they dont get anything for it, I suspect the contract (agreement to buy the thing in question) is no longer valid. That would explain how they can sue to have it returned successfully. It would also explain how the refund might be returnable as store credit.
1 adult bull snake: "Dozer"; 1.1 juvenile bull snakes: Oscar and Phoebe; 3 baby red-sided garters; 1.1 macklot's pythons