is it possible to build an app that works with a Credit Card Reader?
sure, with stripe probably. there’s stripe terminal
You can use most credit card readers if you are well versed in the API connector, but as far as plugins currently I’m only aware of 1 that exists that allows you to use a Stripe Terminal. It’s been a few years since I used the plugin, but I do know that the plugin was for a now unsupported reader.
square is probably you’re best bet
I looked at this a while ago but decided not to proceed but I remember square having a fairly robust api for this