Upload & Sell: Off
First, please check and edit the thread title. Its current form, with a missing r, is scatological.
Second, one way of doing this involves using Photoshop to combine two exposures. The first is of the shirt put on a mannequin or torso form normally. The second is done with the shirt inside-out so the inside of the shirt, with labels and so on, shows. Then the two images are placed in layers, and a layer mask applied to reveal the amount of inside-out shirt that should show under the collar and neck-line of the regular-side-out layer.
Another option is to use special "Ghost Mannequin" or "Invisible Mannequin" forms, such as the one shown here:
Your sample Web page appears to have used a Ghost Mannequin, as I see a white object at the vee of the collar.
The advantage of the first method is that most of us already use photo-editing software like Photoshop, so there's no money to spend, only time and effort. You don't even need a mannequin; a live person can put on the shirt for both shots and be edited out later.
The advantage of the second method is that, while one must buy or rent a special mannequin, it's faster and easier to shoot.