Even if they could prove the Ramseys purchased duct tape and cord, it wouldn't prove they were the murderers - only that the items could have come from the Ramsey home - as did the notepaper, pen, flashlight etc, etc.
I have to say, there's a lot of things I couldn't swear to having bought because they are routine items, but I don't think cord and duct tape are routine items. I distinctly remember buying a roll of duct tape three years ago. We were doing a school musical and the zips in the hired boots kept breaking so we resorted to duct-taping them closed. I can remember going into the DIY store and buying this tape as clearly as though it were yesterday.
It's a great pity McGuckins didn't do itemised receipts.
GhostwheelPyrrhonist