You can buy one of those "netcam", a wired one if there is a router / internet connection nearby, or a wireless one for flexibility in netcam location. Or if you have a PC in your store, you can use any web cam to do the job (though your PC needs to be on at all times).
For wireless netcam, I highly recommend Panasonic BL-C131A. Apart from decent picture quality, swivel and tilt, the best is Panasonic's completely free Viewnetcam service, which makes it very easy for novice to setup remote viewing, like what you want to do with your PDA.
