Definitely avoid this bus unless you can get pictures of the underside of the bus and the wheel wells specifically. The photos look carefully selected to avoid showing the regions they describe as having "some rust". I recently purchased my bus from what turned out to be the Buffalo area and it was described as having "some rust" in the wheel wells, but there were no photos of the area. On delivery of the bus I was surprised to see there was essentially nothing behind the mudflaps and they appeared to be held on only by paint. After I got the plywood floors up I could see how badly the wheel wells and the floor all around it were corroded; I'm going to have to rebuild the entire 10-foot section of floor around the wells.
I also have rust at the bottom of my rear door and top of the bumper, just like yours (at least what shows in the one photo that shows that corrosion).
Generally, reputable auction sales will include multiple photos of the underside of the bus and allow you to draw your own conclusions as to its state of rustiness. Your bus shows no such photos (but in a couple you catch a glimpse of underside stuff which is very rusty) but includes vague language about "some rust" that could be used to plausibly claim you were fairly informed of the state of the bus.
There are many, many buses out there to buy. There is no hurry and no need to jump on an even slightly suspicious one. Like I did.