bit 侧重指微小，一点点，小碎片。
piece 最普通用词，多指从整体上分割下来的东西，可大可小。
sheet 侧重指平整很薄的一张或一片。
slice 多指切得很薄的一片食物。
fragment 指破裂的，不完整的，不可完整的碎片。
scrap 侧重指废弃的零碎之物。
参考例句
1. We want to buy Scrap, Groceries. 我们要采购废料，食品杂货。
2. We want to buy Ferroalloy and Products, Coal, New Materials, Metals, Copper Products, Scrap. 我们要采购铁合金及产品，煤炭，新材料，金属，铜产品，废料。
3. We want to buy Rice, Sugar, Hms Scrap, Rail Scrap, Construction Nails, Milk Powder, Wines Scrap. 我们要采购大米，糖，医学院小片，路轨小块，工程钉子，产乳粉，酒小片。
英英解释
n.
1. a small fragment of something broken off from the whole
2. worthless material that is to be disposed of
3. a small piece of something that is left over after the rest has been used
4. the act of fighting; any contest or struggle v.
1. dispose of (something useless or old)
2. have a disagreement over something
3. make into scrap or refuse
