And then after you've spent hundreds of hours of your own time finding and bidding, which you could have worked, and thousands of pounds on fuel picking up the stuff. Then hundreds of hours trying to make what you've got fit what you want. You'll end up with a finished project you wished you'd spent a bit more money and less of your time on.
You can build a house with zero material costs, you've just got to exchange your time for them. You could work in a shop and exchange your time for some bricks for example.
Or you could nick a brick every time you visit a site.