I havnt had problems yet like this myself even down to purchases of less than a fiver.
Take the refunds through PayPal and when it's cleared leave negative feedback.
Your excuse when they question it is what you say, that the product is still for sale, you have had your time wasted and so you challenge their truth.
It sounds a bit like they are trying to batch purchases to save on postage costs or more than likely have nil stock.
They leave adds running all the time as otherwise they keep having to pay new listing fees.
Suppliers like RDG do this but they take the trouble to amend their adds with a massive purchase price which you read as out of stock for the time being so no relisting fees.
Looks like you may be dealing with a supplier that hasn't done this.