Yes, Powermax is "legit". They are an authorized dealer of new and used macs. If their products are refurb./opened - they say that.
If you want to buy a refurb mac, Apple's website is generally the best place to do it. However, since Apple has so many physical Apple Stores now, for most states you'll have to pay sales tax. Unless you live in Oregon (the one state w/ no sales tax) or a state with no Apple store, you'll pay 6 to 10 percent in sales tax for anything you buy from them. (If you live in Portland, Oregon, the cheapest way to buy the latest model of any mac is almost always going to be to buy a refurb unit through the online Apple store.) With Amazon and many other online only stores - for places that don't have physical stores or distribution centers in the state where the customer lives - there is no sales tax. (That goes macmall, powermax, etc.). So if Amazon is selling a white macbook for $949 and you live in, say, Tennessee - the price you will pay if you order it from them will be $949. If Apple is selling the same computer for $999 and you order from them, the price you will pay will actually be closer to 11 hundred bucks. Even before the sales tax is factored in, these guy's have cheaper pricing than Apple because they buy PCs from Apple in bulk, at a discount, and then they sell them back to consumers with very little profit margin. Sometimes their PCs are even sold as a loss leader (that is, they sell PCs at a loss to attract sales of other product- just like grocery stores often do with things like bananas). They generally make most of their profit off peripherals and other hardware and software. Apple likes it this way because... well... they just do. (It's just how it is.) I suppose there are reasons a consumer would choose to buy directly from Apple instead of from a reseller, even though Apple is charging a lot more for the exact same product (just as people might prefer to buy an airplane ticket directly from an airline instead of using a website like Travelocity) - but I don't really see any reason in doing that.
Several resellers current prices for all current Apple PC models are laid out on a helpful page updated each day at appleinsider:
http://www.appleinsider.com/mac_price_guide/ . Keep in mind the best deal is usually only best if you don't have to pay sales tax. If you do buy directly from Apple, whatever you do, don't pay them for a RAM upgrade. Buy extra RAM from someplace else (like OWC) and install it yourself or take it to a someone else and have them install it for you. Apple's RAM pricing is nearly criminal. Ditto with Apple care: Don't buy it from Apple. Amazon generally sells it for a 20-30 percent discount.
