A preview is a few seconds long, but it answers most questions before you buy.
What to look for
Is the camera still? Is the light good enough to see the scene? Does the preview show the actual setting and the actual performer? Is there sound? A preview that shows nothing of the scene is a warning sign.
Length and price
The store page lists both. A short clip with a strong preview is usually a better buy than a long clip whose preview avoids the point.
See the most-watched previews on the front page, or read free scat or stolen?