Why? An order you didn’t really need was canceled due to massive customer over ordering that their system wasn’t built to handle?
Any online system would have a view of stock levels and decrement as orders are placed, then make the product unavailable when all stock is sold. Over ordering cannot happen other than odd ones or twos if the stock count is out.
It sounds like they deliberately or accidentally made stock available that they don’t have, or at a price they don’t want to sell for.
T&Cs usually protect the retailer so it’s at the buyers discretion to decide whether that’s enough to boycott a retailer – the reality is that most of our principles can be equally matched by the desirability of the product and the level of discount being offered in the future.