Hi

Have you looked at the learning center guide information?

Unknown said:

We buy kit which consist of different parts. And sell each part to customers

Use a recurring, Build from Item Assembly transaction, to record taking a quantity and value of kits and assigning a quantity value to the component parts. 

Additional Costs would be for the labour or supplies (i.e. packing material) required, or the supplies could be made part of the 'in'

 part of Item Assembly.

Don't use a Bill of Materials with a negative quantity to 'un-build' the kits - the costs may not be correct, since they are calculated on the current, per-item Last Cost of the individual items, whether building or un-building.