> You can already easily just put stuff in your pocket or ring expensive items up as incorrect, cheaper things

This is actually already somewhat solved. The other day I was at a Safeway and one carton of milk had an unreadable label so I scanned a different carton and put the unreadable one in the bag instead (same product). The system showed me a video of me doing that, highlighting the fact that I didn't put the item I scanned on the bag (!) and asked me to wait for an assistant. Pretty impressive.