There is nothing to enforce a seller to send a paid for item, have you paid?
This is bad selling did the seller give the reason why they wish to cancel?
You can of course disagree to the cancellation if they go ahead and select Buyer requests to cancel you can contact eBay Customer Services they will place a damaging defect on their account. They have few reasons for cancellation, cancelling out of stock gives them this defect.
@robhaw_47