Other states have a sales tax ,which is a percentage charged

主干：Other states have a sales tax...
后面整句都是由which引导的非限制性定语从句（其中which you buy是第二个定语从句修饰item）——
...which is a percentage charged to any item (which you buy).
句中的charge是动词“收税”的意思,而不是名词,用过去分词表示“被征税”,为方便理解,将语序稍微调换——
...which is charged a percentage to any item (which you buy).
整句翻译——一些国家有买卖税收,这种税是按照（你买到）物品价格的百分比来征收的.