30% off= 打七折
bargain / haggle 打折
I will buy it right away if it were cheaper.
如果便宜点，我马上就买。
Hey, don't try to rip me off. I know what this is worth.
别想宰我，我是识货的人。

Buy one get one free
买一送一
exchange/ refund :
when we want to exchange the goods for another size or another color we call that exchange.
当你想要换衣服的尺寸和颜色的时候。
When we want to return the clothes or the good back and we want the money back we call it refund.
当你不想要你买的衣服或其他的东西，想要退回的时候。