The company, operated out of a basement office in Cosner's Hagerstown home, has four full-time and four part-time employees at this point, they said.
That could soon grow to 20 employees, said the Hagerstown natives, who plan to open a second Internet store this fall.
Complete Health opened earlier this month with about 1,000 items, ranging from vitamins to massage chairs, said Cosner, 36.
About 5,000 more products have been added since then, she said.
Given the dynamic nature of the health and fitness industry and the freedom of not having a physical inventory, Cosner said she expects they'll constantly be adding new and newly discovered items to the mix.
"There are wonderful products today people either haven't heard about or don't know how to find," she said.
Take, for example, a menu-planning software program that customizes a week's worth of recipes to the right amount of diners then makes a grocery shopping list of all needed items.
You'd find that by clicking over to "Super Software."
It's also included in "Great Gifts," which offers a variety of gift suggestions based on the price range you choose.
The department structure makes it easy for customers to shop the site (http://www.completehealth.com), said Cosner, who said sales have been "very brisk" in its first weeks.
Customers can click on the various departments - including "Book Bazaar," "Sports Nutrition," "Kozy Kitchen," "Hot Deals" and "Massage Therapy" - for a choice of more specific subcategories.
Products are displayed in an easy-to-read spreadsheet format, including the product's name, a photograph, a short description and both retail and selling prices.
Customers can get a close-up of a product and a more detailed description by clicking on the item.
Desired items go into a virtual shopping basket, with a running total of its value displayed on every screen.
Online payment - either by credit card or CyberCash - is preferrable, Guy said.
However, they'll take orders by telephone if customers are skittish about inputing their credit card information, despite the assurance it's kept secure with a special encrypted system, she said.
Guy and Cosner see tremendous potential in both running their own Internet stores and setting up retail sites for others through Sister Web Sites.
"I think the world is ready for this," said Guy, 31, of Greencastle, Pa. "We're excited about the potential."
The company plans to start capitalizing on its popular Diet Talk Web site (http://www.diettalk.com) - born in February out of the sisters' struggles to lose post-pregnancy pounds - by selling advertising, she said.
The site averages 400 to 500 hits from around the world each day, she said.
The company plans to debut its Christian Central site (http://www.christiancentral.com) - offering one-stop shopping for Christian merchandise as well as a chat room featuring guest hosts - on Sept. 15.