I am familiar with both sportlots and COMC. I have used both. The key thing about JustCommons is that you are buying from a single seller and only pay one shipping charge. Shipping is free on any orders of $10 or more. It is a great way to fill out sets on the cheap. I would like to find something similar for football.
Wants: Topps (1965), Topps Heritage (2014) and Topps All-Star Rookie Trophy cards