If you want to show themes to potential clients before purchasing the theme, show them the live preview of the theme which you can find over here.
But i already know, you ( or anyone else ) don’t like to do that, because then clients will see it’s only 40 or 50 bucks. And you want to sell it for more, which is okay.
But, since there are really beautiful themes you can get for only these prices, why not buying a couple a themes in front? I mean most people who are doing business like that can easily afford it since they probably sell a site for 10 to 20 times the price at least. Choose some themes you like and think you can use someday. Set them up as a preview at your own server so you can show it to the clients.
A small investment.