What are the difficulties you face in selling a big project and why?

you cannot sell any project without being able to convince the company that the realised benefits are worth such a heavy investment, and since you may not have a tangible thing to use as a benchmark, it would take a while to create a case for your project, and you may need to try several times before you are awarded such project

Big Projects mean huge investment (High Budget), longer completion period (Long Schedule) and risks (Higher Risks). These three factors act as deterrent to attract sponsors. 

As a project management professional, it is advised to break-up Large projects into a number of smaller Projects or Phases and sell each of the smaller Projects or Phases to sponsors in a phased manner over a period of time.