Or even duff goods that eBay and PayPal then side with the seller on.
As I understand it if the goods are not labelled as gift, as they ought not be if you bought them, then customs may open the package to see what tax is due. No one gives you a straight answer because no one knows for sure what will happen to your package.
Free postage from China ? Either they have a UK depot, or you are paying a lot over the odds for the item to cover it, or the item is so lacking in value it still pays them to flog it to you international post free and make a profit.
My advice is to go for a UK seller if you can. Nothing is guarantied above board but you have next to no chance if a purchase across the world goes wrong. 'UK seller' must be worth something on top to you.