When we buy items from EBAY,How do tax apply?

It depends on the seller. Some people will say in the description that tax applies but it might be only for a few U.S. states. If they don't say anything about it, that means the only extra costs would be shipping. Before you buy something from eBay, just look over everything the seller says and make sure it doesn't say anything about taxes where you live. I haven't seen any items that tax people in Sri Lanka but there might be something so just double check.