Sometimes the price of the mall includes setting up your page, scanning your work, and doing all the HTML to put your gallery on the web. If you are not computer savvy, or don't have the time to do the work yourself, this, if priced fairly, can be an advantage. The downside is that the sites often follow a strict template - forcing your work to fit into their formula. A limit is placed on how many pieces you can show, and you usually must pay an additional fee to update your pages with new work. Be sure you like the look of what they have done for others, before you commit to their design.
Some people are wary of putting their credit card information out on the Internet.
A secure site uses encryption to keep the credit card data safe. How well the secure ordering system is designed is something to be evaluated closely. I've been so frustrated by some sites that I've tried to buy from I left before the purchase was made. One merchant reported that his sales doubled when he added a toll free 800 number to his site, giving his customers the option of calling with questions and ordering the old fashioned way. A mall that forces customers to order only through the web site is not a good idea.