When it comes to merchandising a group there are two sizeable obstacles. One is the production of the merchandise itself. The other is the joint issue of marketing and sales.
Although the Internet, in general, has made the manufacture of custom goods easier, it can still be costly to produce and keep an inventory of t-shirts, much less bumper stickers, coffee mugs and other paraphernalia. Most groups can't afford to go with a local vendor for more than a set of shirts for the group members themselves, and though online producers will often cut a deal for bulk sales, it still leaves it up to a group to front some serious dough.
After the merchandise is ready for sale, there's the matter of making fans aware the product exists, making them want to buy it, and the actual procedure of selling it. Lots of groups will succeed at selling merchandise during intermission or right after live gigs, but in a plastic age, how many fans will really have cash on them to make that happen. Setting up credit card or Paypal sales through the group website is certainly possible, but similarly challenging if the group does not already have some expertise in this area.
Enter a site like Cafe Press that facilitates easy screen printing on a range of clothes and personal items, and provides users a ready to use storefront. A resource that takes care of production and sales without any monetary obligation from the group is truly a can't-miss resource.
Of course, the one piece Cafe Press will not take care of is marketing, so it is up the group to push web traffic to their store by any other means possible--a link on their website, of course, but also pushing the link through remidners on Facebook and Twitter, and making sure to mention it of live shows.
Selling merchandise online is a form of networking because it puts your group out there and makes it accessible in an important way to fans and to other groups. A fan who can walk around, proudly sporting the logo of her favorite a cappella group is a committed fan, and that's exactly the kind of audience groups should strive to build.