Those battery eliminators are legitimate products. Canon makes them for some of its PowerShots. The AC to DC converter takes wall power and delivers the appropriate voltage (and adequate current) to match what would be delivered by the specific battery or batteries. If the only way to power the device is by inserting batteries, these battery-shaped things can be very convenient. If there is another way to power the device (e.g., with Arduinos), then only the AC-DC converter is needed. An AC-DC converter typically costs just a few dollars, so those $50.00 battery-shaped things are quite a scam.