You shouldn't have bought it if it wasn't eating. Ribbon Eels are hard to get to eat, not to mention that you need to make sure you tank has plenty of rocks, for hiding spaces, and has a good lid, as Ribbons will climb out of the tank.
Try feeding it pieces of krill, small fish or small shrimp on a stick. Try feeding live ghost shrimp if you can get them. If the eel wouldn't eat from the stick your best bet is to leave some food on the bottom of the tank or live ghost shrimp. Hopefully the eel will start to eat. It's very hit on miss, the only good thing it that they can go for quite a while without eating.
You may also want to keep the light down or off until the eel settles in, not to mention most eels hunt at night.