We have purchased a marketplace theme and our designers would like to change the design.
Is it possible to get the product page and product list in PSD?
or how we can change the design?
Depending on the theme in question the author may not have PSDs - it’s not uncommon for authors to go straight to compilation stage when building their site.
Your best option would be to ask the author of the theme
Also you certainly do not need the PSD to be able to alter the design of the theme/template. That was the thing maybe ten years ago. Nowadays you can simply make any changes using the object inspector in your browser before you move them to your custom stylesheet.