"but where am I supposed to enter the cost of the purchases?"

Think of this as Investment. Not Cost.

"in the cost of goods sold section"

If they are sold, then all costs associated would be Cost of Sales for those units sold, including improvements (repairs) and perhaps financing (flooring).

"but which line? and what other expense can be claimed for them?"

Are you familiar with inventory-driven businesses? Does your client even know how to handle and report to you their inventory data? Are you familiar with the industry, which has some specific quirks? They are working with Titled and Registered property, trade ins, financing of the buying and the selling, etc.

You want to be mentored in this, to learn from it.