First, that is a good good deal.
I can't speak directly about the Duro as I use a Ceros instead but I'm betting there is a tube in there. The tires themselves are tubeless but unless some past owner sealed the rim for tubeless use, it's probably got a tube. Most if not all are using tubes when using an ATV tire on the TW rim.
Now, I have a Ceros mounted on a modified TW hub (the trailer wheel mod) and this setup is tubeless.
Even though the tire may not be DOT approved (I don't have personal knowledge of that with a Duro) many ride on the highway/streets with the Duro. Forum member Fred is a good example. He rides his all over the place, highways, logging roads, single track trails, hardware store aisles, ok maybe not hardware stores. However, there is nothing wrong with using this as an off-road package only is not a big deal either.
Depending on the size of the tire, you may want to use a larger than stock sprocket if riding off-road steep trails. The larger tire size I have to use (26x9x14) is a dog off-road with my Ceros. I use a 13/54T combo on street and normal off-road use which isn't too rough, i.e. any steep spots. On my last ride with the forum group in the Sweetwater Mtns. I changed to my 60T rear sprocket as I knew we were riding in steep terrain and I was gonna need some oomph for the hills.
I agree with the owner about not worrying about the swing arm. The one you're getting is 8 inches wide. Mine is 9 inches wide (only size I can get in the Ceros) and I do have to set the tire back in the swingarm for clearance with limited future chain adjustments. I think your lucky, this is a good deal.