I am leaning towards Schedule C based on information provided-

1. TP has done this multiple times

2. TP bought the land with the intent to build and sell.

3. TP acted as a general contractor and provided the construction plans- "His participation involved finding subcontractors for the plans, pretty much".  TP apparently monitored the work and approved payments- both progress payments and final.

4. The fact that he used a realtor to sell the property does not change the character of his actions above.