If you try out a projector for 2-3 weeks and then return it, you have cost that dealer a lot of money. He will have lost the 3% credit card fee, the shipping cost if shipping was included one way and the reduced selling price of an open box projector with several hours on it. Here in the US, that loss would be over $500.
That would be his problem, right ? If he doesn't put any conditions to the 30-days, then he is obviously OK with this possibility. In any case, I would say from the beginning that I am not sure about the picture quality and that I would like to try it before I spend €6000. I wouldn't pay by credit card anyway, and would never have a €6000 projector shipped, I would go and pick it up.