My thoughts exactly. The poster does not know what he bought.It's a belt/disc sander. It could be a 4" x 36" or a 6" x 48" belt, and a 6" to 12" disc. There are several brands of those machines. The table can be mounted for the belt or the disc, and can have an adjustment capability to be set at varying degrees to the disc or the belt. The belt mechanism can be set flat (as in your picture) or stood up vertical.
Is that what you were asking "What am I?"
If you said "What brand", might have been simpler. It may not be a bearing. There are things to check first before going that route. There should be a tracking knob for the belt. That might just need adjustment. You could also check for debris under the belt or on the rollers, which could bear on the belt causing it to pitch to one side or the other. The belt itself could be old and out of whack.Lol I know what it's called, how to operate it.
I'm attempting to ID it so I can find parts. It works fine except the belt wonders due to a bad bearing on the non-drive hub.