It happens

Just as in a B&M supermarket, you may go in there wanting X brand bread but if the X brand bread factory did not deliver any, the supermarket can't sell it even though they usually sell it and have it advertised as being for sale

At least this seller told you and offered the refund straight up front, some on the dodgey side would have tried to tell you wait for a few weeks and they *promise* to send it or offered a different product at twice the price

I do not know your seller's 'set up' but unless they were a large company with a warehouse or unlimited storage it is likely at some point in time a supplier is going to fail them