Would you buy this frame with a rusted bottom bracket like the one in the photo? Would the other tubes (downtube, stays) be rusted also? I cannot tell you if the rust is superficial or not since the frame is for sale at a website.

It looks a bit superficial based on the condition of everything else but you can never tell from one picture.

There are generally holes from the seat tube to the bottom bracket. Also often in the bottom of the bottom bracket.

The threads look good. I wouldn't worry about it too much.

This bike model does not have any drain holes in the bottom bracket.

I would expect internal rust in the chain stays and probably the down tube as well. Do you have the time/tools/aptitude/space to properly deal with internal rust? If so, no big deal, although it is a bit of a PITA. If not, move on.

At the very least, I would be looking for a hefty discount.

Looks like a Columbus Schwinn Tenax frame. If rust isn't coming out in large flakes, I wouldn't worry about it too much, especially if it's only $50 or less. Cleans up with a brass brush on a drill and some WD40.

Looks pretty typical to me, and the fact that the threads are clean suggests that the rust further inboard is entirely superficial.

There is no problem, as near as I can tell from the photos. I would buy the frame and not worry about it.

I'd put a brass wire wheel on a drill and go to town to clean it up.

The frame was a Schwinn Circuit. I pass on the frame due to price.