Yes you have to be very careful with square or Inuit (quickbooks) gopayment type swiper, It is very easy for someone to call up and say it wasn't them that made the charge and its on you to prove it otherwise. Also if someone has a job that allows them to see peoples credit card the will take it down and try to "prepay" their appointment. It might also be dad's or mom's card.
As soon as I hear "prepay" I can't see my hand in front of my face for all the red flags blocking my vision. lol Never enter the card manually, Its 95% sure they don't have permission to use it.
The square type swipers are ok for a regular client that you have built up a relationship with. Just don't use for a unknown new client. Another flag is if the number is a weird number like the phone apps use. They won't use their own number so makes it harder to trace back to them if you pursue the charge back.
I have switched to a TD POS (point of sale) terminal. Reads the chip, they have to agree to the charge and enter pin. All on the up and up and then the onus is on the card owner to prove it wasn't them that used the card and they provided their pin to someone else which automatically makes the charge back almost impossible for them.
The most important thing is if it seems to good to be true, guess what..... Never have I had someone swing by the house to drop off cash to prepay for an appointment for later in the day so I don't expect someone to do it with a CC either.
Be careful out there. XOXOXO Riza