You can get an information about template available in a Templatemonster database and have them displayed at your template shop using 4 different methods:
XML Web API;
By integrating scripts from a ready-made reseller shop engine into your website design.
Apply to Web API User Guide for information on how to get the data from Templatemonster database and get products (this refers to web templates thumbnails, screenshots, search facilities, order status forms, shopping carts, products description, etc.) displayed at your website using Web API.