If there's no EXIF information then it's likely that a photograph is not an original from a digital camera. Some manipulations must have happened.
Detecting changes in a photo is possible with proper forensics software. This is helpful when the image itself isn't showing any traces. The software works by doing pattern recognition, since digital camera's tend to have specific patterns in the pictures they create. Most image manipulations will change these patterns which can be recognised. The software should even be able to give some indications what kind of manipulations have taken place.
Where you can find this software? Well, the FBI, CIA and Interpol will be using it. Don't know if there are any commercial products available but be prepared to pay an arm and a leg for it. (I think it would be real expensive.)
Most image analysis software will be created to recognise shapes and objects. But the kind of software you need is something which can trace manipulations. It's not much more complex than recognising objects, but almost no one is interested in this kind of software. (Except of course to detect if an image is real or fake.)
It is bad that the seller is touching op those images? Maybe he just sharpened the image and removed some things from the background.