They should not do this and it is against Ebay policy.
However, there is no way to force a seller to complete the sale and send the item.
You can leave negative feedback if you wish and, if the seller used a false reason for cancelling, you can report them to Ebay.
Personally, I would just find a better seller.
"There are two ways to be fooled. One is to believe what isn't true; the other is to refuse to believe what is true.”
Søren Kierkegaard, Danish philosopher (1813 - 1855)
