Have the books not moving, then have one book fall from the shelf. When someone bends over, picks it up and begins to slide the book back where it belongs THEN is when the books all begin doing their trick!
The mind might slip into a mundane mode, forgetting to be wary and also doing a physical act like merely picking up a book and putting it back also relaxes and distracts, making the moving books ,when it happens, more effective.
Try a thrift store for the ice cream motors, I see LOADS of them there! Usually around $5 or less. You may have to buy the whole thing, but if you shop around (and are lucky) you can find one. They can be a bit noisy tho....
This does look like a neat prop, and I have seen it before. I may have to try one this year!
Ok maybe it seems like a lot of money but to whoever made it it is probably what they think it is worth. To be honest I think it works a little better then the others I have seen. But, to answer the question, no I would not pay for it. However that is not saying much because our typical budget is 25-50 dollars a year. Plus it is so much more fun to make stuff your self.
On a semi-related note, my buddy just built a hinged bookcase to go between their basement family room (which has a monster theme) and utility area. Right now it holds in place by means of a ridge on the floor that the rollers go into, but he is doing a latch mechanism that operates by moving a book. He REALLY wanted to do something with a candlestick ala "Young Frankenstein". It is the coolest thing!