Getting a mag strip reader (MSR) isn't hard and a USB can be found for under $20.
So you read out the tracks and get the data.
You would need to input the amount. Keypad and 7 segment display should work there. While smart watches aren't ok, I suspect that the some electrical cash register is acceptable... and then...
Wait a moment, if they've got an electric cash register they've got the ability to have a MSR.
Anyways, let's assume that the 7 segment display for input of the amount is acceptable. If not... Nixie tubes?
Once the amount to charge is entered, and the card is swiped through the MSR, a receipt printer with the result is used (though they might have issues about that since thermal paper receipts often have BPA https://www.pca.state.mn.us/business-with-us/bpa-and-bps-in-... )
Anyways, result of the transaction through whatever payment processor happens and is sent back to the pi (no screen) that then prints out the result.
Sending credit card number to a payment processor from a MSR is no more difficult than sending it to the same processor from hand entered (for example, online). You might have a higher fee rate, but it's not difficult.
Hmm, possibly but I am thinking there are some certification requirements e.g. and you have to be able to use some API whether that is hypermedia or otherwise, which again credit card companies must agree to let you use.
You could maybe bypass them with a third party API, I'd have to do my research to see what the gotchas might be though.
I wonder if there are some issuers out there who still have some active legacy backoffice process for processing transactions with carbon copies from imprinter machines
You could design one without, which would be interesting, but good luck integrating with the major cc companies.