ShopcartUSA is basically a redirect site that will go out and find products that are for sale online. If you want to find out if that $800.00 GL2 is from a reputable online seller then you need to take the name of that seller and plug it into http://www.resellerratings.com. There you'll be able to check that company's online rating and read reviews from actual customers that have done business with that company.
Places like B&H Photo will have an exellent rating while less reputable places will not. The short answer is if the price seems too good to be true then it usually is.