So to prevent inconvenience in both parties here are some tips:
For Store Managers:
1. Have a regular inspection on all items in the selling area whether they still have barcodes or not
2. Always initiate pre-scanning of barcodes most especially during sale events
3. Use of high-quality stickers for barcodes is a must
4. Place barcodes according to standards
5. Always set-aside items that have no barcodes like those returned by customers
6. Source out new POS programs where Cashiers have access to the store inventory where she can access then clarify all problems relating to barcodes. (Some department stores already have this system.)
For Customers:
I know this is not your job, but a little check on the items' barcode before proceeding to the counter could save you hassles in the counter.