I don't have a Raspberry Pi, but I can guarantee you that if it has the X server and SSH that it needs nothing more than the IP address where the point of sale client application is running to turn any touchscreen display into a point of sale terminal. The other hardware components include things like a Wireless Access Point, WiFi receipt printers, cash drawers attached to these receipt printers, WiFi kitchen printers and WiFi credit/debit card terminals. There are some very affordable touchscreen displays these days; we've obtained some out of China for about 130 quid. There's absolutely no need to buy, indeed no point in buying, any hardware built specifically for point of sale. That is merely a perversion that some of the copycat point of sale companies engage in to lock people into their software. Look at this $250 touchscreen, for example. http://www.amazon.com/PX2230MW.....B003B47BZO
Add a Raspberry Pi to the back of that, plus a printer and cash drawer and you are in business. You could have people with tablets taking orders at the table, too, and all you have to do is give each tablet the IP address where the client application is running. It can be local or in the metro area or anywhere on the Internet. The client app was originally designed, and runs, in less than 8 Mb, on a 67 Mhz processor, and it would certainly run fine on the Raspberry Pi if necessary. The graphics are embedded in the source code so it's a very responsive GUI, written directly to the Xlib primitives for speed. There are several locations in West London and Reading I can send you to if you'd like to see this software in use, including on tablets.