You know…a commercial and personal license of the same project might not be doable, but some kind of sample would work. If the product is large enough to provide samples, have a download link. If not, have a packaged game, like how generic shooter has a downloadable demo, that shows off what it can do. Other products focus on one or two models (often a really really really high quality one); if it is a gun, they could set up a packaged game free to download that has a single level with a shooting gallery and displays of the weapon.
This would especially be good for environment assets, people can walk around looking at what it could look like and not through a youtube video.
This isn’t something that has to be a part of the marketplace, but just common practice with content creators.
It might not keep anyone from saying the prices are too high, but no one can say they didn’t know exactly what they were getting.
Unity has the Unity Web Player which allows for previews of assets. They’ve stated that they are working on producing a similar thing for Unreal, but no date on how long that will take to produce.