Buying a Site - Advice Required

I am looking to buy a site with a view to building a 2,500sqft property.

What should I ask the seller of the site? I am familar with terms such as "FPP", "being from the area", "required occupancy for a number of years", "serviced", "percolation test"

But what else is there to know??

I suppose I want to feel confident when approaching the seller...