Barb's idea of a sign saying 75% off is a good one. I certainly would have bought fabric at your price.
I was at a garage sale last week where someone had taken a number of cd's without paying for them. The seller didn't know what to do, but had taken down the licence number. I encouraged her to call police and held down the fort for her while she did so. Unless the situation is dangerous, I would definitely confront someone who is stealing items that are already being practically given away because of low pricing.