I along with my friends are starting up a second-hand book store.
So, we want a portable mobile credit card reader, which is easy on the pocket (as we are a startup, we would be able to shell not more than $100)
As the store is a portable set-up such that it can be moved easily from a place to another place, a nice, portable and durable reader is preferred.
No issues with the compatibility, but an Android compatible device would be preferred in case of a specs tie, as most of the team members have an android device.
From the comments, I came to know that it is called a "Point of Sale" machine, just in case it might help while answering.