It´s a good question. I think theme developers are realising the need to play down their branding, because after all this is a marketplace where we buy and sell. I think if you show customers a demo, you are basically taking away value, because you are telling them it is easy to put together. I would go the screenshot route and explain features to them. I´m sure a theme developer won´t mind you using a screenshot of their work if you end up buying it from them.
I don´t tell people they are themes, just as the supermarket doesn´t tell me the actual price they pay for a tin of beans. If anyone was to confront me, I would say ´you´re correct, but do you have several weeks to learn the knowledge to put one together successfully?´ Obviously you can´t go too over-board on the price, but that is a judgement you have to make.