I have purchased several pieces of machinery through IRS Auctions (sorry Phil!), as well as picked up and transported a large lumber purchase for other NCWW members.
Yes, they are legit.
Be aware that there is a buyers fee on top of the auction price, and that you need to pay for your purchase via money order, wire transfer or certified check within 48 hours of the close of the auction. You will not receive the pickup instructions until they have been paid, and they will not let you "pay when you pick up".
Additionally, they typically do not cater to "Joe Woodworker with a pickup truck" in terms of loading out; they target industrial purchasers. Exceptions can be made, but usually you need to show up with a flat bed trailer that will allow your purchase to be forklifted onto it. If you think that you can show up and load up a bunch of lumber into the back of your pickup by hand, they "may" turn you away (and keep in mind that you will have already paid for your stuff).