Please help, ebay seller says item out of stock after i paid and they don't know how to cancel it

[tactual]

I bought something last night and paid for it but got an email this morning from the seller saying this "

Oh I'm sorry I should have checked earlier
It is sold out so I have to cancel it. Because I'm the beginner I don't know how to cancel it and refund could you cancel it?"

What the heck? This is so unprofessional of them, as the buyer who has alreday paid PLEASE walk me through the steps to getting a refund because this is the first time this has happened to me and I have no clue what to do!