IMHO for your POS app with 10-20 concurent users, it is not necessary to use a DataBase server - native FoxPro tables will do the job. VFP is more effecient than some of the current DataBase servers when handling not extra-large amounts of data.
Also you may split your app in two parts - one that is running on the server and will do some of the preprocessing, and other that is running on the client.
Note that this complicated design is NOT a requirement, just an option.

In the other case - you will have shared data files, that you access through the network. Think about making local database containers for the views, if you have performance issues (the last tip is from the book "Visual FoxPro Certification Exams" which you can get by Hentzenwerke publishing)