There are ways to do this if you have the technical capability but it is not appropriate to do so and am pretty certain that they would heavily violate copyright, as would screengrabbing unless you clearly state it is not your own work. Besides is it not better to show them at least mock ups (obviously working version is not possible) with their content/branding?
Why not share a link to the developers site e.g. without the Themeforest banner? That would be the easiest way.
The only possible reason I can see that anyone (not aimed at anyone in particular) would want to show themes on their site with no affiliation or links to Themeforest or at very leas the developer site is if they were trying to hide the fact they were a template. This is unprofessional and dishonest, hence why it is not easy to do.
If for whatever reason you want to do this then you really will need to buy the file sin question. Hopefully that set sthe record straight and helps to explain the reasoning behind it.