once they start using writable RFID chips* we're going to be seeing some nasty cases.
as you're paying for the stuff, it's "tagged" paid.
The stuff isn't changed, the entry in the database is marked as paid. If you are thinking that a thief is going to be able to pick something off the shelf, rewrite the tag and mark it as "paid" and walk out the door, then I don't think it works like that.
Let's compare barcodes with RFID. The sheer number of different available RFID tags is so large, that manufacturers could put a different number on each individual widget that goes out. In the past and present, each copy of an item gets the same barcode, lets say for example that 6789054321 is the barcode for Levis Boot Cut Jeans every single pair of Levis Boot Cut Jeans will have the same barcode:
6789054321.
In RFID land, you have so many codes to play with, that you could just say "Every RFID tag between
6789054321000000000001 and
6789054321999999999999 will be Levis Boot Cut Jeans" and then you could have 1,000,000,000,000 pairs of jeans out there each with its own unique tag. You ship a box f jeans to walmart and they get scanned coming in the loading dock. Each article has its own entry in a vast database. They are all marked with a particular status "In the stockroom", "On the shelf", "Off the shelf" (the shelves will have short-range readers*), and finally, "Paid for". Anything going out the front door is scanned and identified. If it's status in the database is anything other than "paid for" alarms go off.
* I used to work in a business tied with retail research and development. Spent a lot of time in futuristic mockups of retail environments. Multi-million-dollar projects built solely for purposes of figuring out how to convince a shopper to spend $0.50 more on razor blades or what-have-you. RFID-enabled shelving exists and is probably coming to a store near you. I was shown a demonstration where as I took small, shoplifting-prone objects off a shelf, a handheld computer nearby ticked off current shelf inventory, warning fake employees to restock. I also was shown how if I took more than 5 items off the shelf at once, the same computer lit up red with "Potential Shoplifter" and a picture of my face taken from a camera I couldn't even find after I knew my picture had been taken. This was 6 years ago.