"All my purchases are being cancelled after the payment for no reason."
I've read that some private sellers are cancelling orders because they listed the item using Custom Postage, and it's only when the item sells that they discover that ebay have switched it to Simple Delivery. Some sellers don't want to be forced to use Simple Delivery so they cancel the order. You'd think they would contact the buyer to let them know though.
However, this MAY NOT be the reason, particularly if you've bought from a business seller as Simple Delivery doesn't apply to them anyway. As advised above, the only way to find out is to contact Customer Service.