Pretty much all card readers use a screen, even if it is just a cheap green one.
You could design one without, which would be interesting, but good luck integrating with the major cc companies.
You could design one without, which would be interesting, but good luck integrating with the major cc companies.
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.
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.