I read the entire act.
The act is designed to limit the purchase by businesses of materials stolen from homes and businesses. These items are copper, wiring, aluminum, architectural details from houses, etc. The purpose of paying by check and getting identification from the seller is to help track down the thieves. Yes, it is possible that some over-reaching DA or judge could include a poor little old grandmother selling stuff at a garage sale, but I'd say the chances are slim. Besides I'd pardon the grandmother.
To clarify: FF steals the copper tubing from various central A/C units. He takes it to XQ scrap yard. Before, XQ would buy the copper for cash and FF would go off scot free. Now having to show ID and paying in check establishes a trail of evidence. If FF makes a steady stream of sales, he's opening himself to not only local police action, but questions from the IRS. I'm guessing the IRS is going to ask for a 1099 for any purchases over a certain total amount.
I read of one poor guy who had his architectural arches stolen from the house when it was being repaired from Katrina. He bought the arches back 3 times from various salvage yards. Yes, they were his because they are unique. Finally he engraved the address and his phone number on them to make it easier for him to be called by the companies.