1. Hey, don't try to rip me off. I know what this is worth.

别想宰我，我识货。

Rip someone off 意为某商店或商贩企图宰它的顾客。买东西最担心被宰，行家都要先造出声势，用这句话镇住卖家。如果真的被狠宰了一下，可以说，"I was ripped off ."或者"I was cleaned out."

The shop tried to rip me off, but I taught them a good lesson.

那个商店企图宰我，结果让我教训了一顿。

2. Can you give me a little deal on this?

这能卖得便宜一点吗？

Can you give me this for cheaper?

能便宜一点给我吗？

3. Is there any discount on bulk purchases?

我多买些能打折吗？

Give me a discount.

给我打个折吧。

Bulk purchases就是"大量地购买"，等于buy something in bulk。

"Discount"是折扣的意思，平时在商店里常出现的表示打折的牌子是on sale。专门卖廉价物品的商店叫bargain store，店里卖廉价商品的柜台可以叫做bargain counter。