when a credit card doesnt scan and then they put it inside a grocery bag and scan it and it works.... what is?

"The magnetic strip on the back of credit (and debit) cards can become slightly "smeared" over time. This smearing moves some of the magnetic particles away from where they were originally. Usually, this doesn't affect the data on the card, but the smeared particles can create small discrepancies in the data stream -- sometimes enough that a card reader won't read it anymore. When your card becomes this smeared, you should obtain a new card. While it won't work for very smeared strips, using a plastic bag separates the magnetic strip from the reader just enough that these small discrepancies in the data stream are averaged out. Essentially, it improves the signal to noise ratio."

ive never had this happen to me - it sounds a bit dodgy, they could be consealing a swiping device in the bag and stealing your card details.

I think it clear debris from the reader.

thats called skill