A few years ago, I saw a Paul Durand-Ruel exhibit at the Philadelphia Art Museum. Paul was alive during the impressionist period and his contemporaries included Manet, Monet, Degas, Renoir, etc..
The impressionist movement was not catching on and people were opposed to seeing brush strokes on the canvas. He started buying up art at a price higher than what was asked. He created a false demand for this work so he could sell it at a higher price. He was a leading art dealer at this time. We have him to thank for many of the masterpieces that we can view today.
If that doesn't suggest that art prices can be manipulated, nothing does.