J2Store has a free module to display products ordered by some parameters like Latest added, Cost, Best-selling that can be download via below link:
Anyway if you need this sort, open below file and go to line (196) :
case 'desc': $sortby = 'p.j2store_product_id DESC';
case 'desc':// sort based on modified date $sortby = 'p.modified_on DESC';
Now in module edit page, if you select “latest product” the list will be sorted by latest modified date. Of course you can use this change for any other case like art_desc as you like.