"add to cart" in responvive clasifieds theme

Hello
I want to create a marketplace where small stores from my neibourhood will could put their products in my marketplace.
i like how works Classified theme with “add listing” to make the registratión and put the listing in the web. But…. the final botton is “Contact Author”. I would like the final botton will be “add to cart”.
can this be possible?

thanks