funny, I had the hideout map from the start, and the bandit didn't appear there in the basement. had I not seen your image and message, I wouldn't have even known since I never looked around the city's perimeter.
is it because you're supposed to lose the fight against the bandits you encounter while roaming the desert aimlessly? Because I kicked their faces in. That's the only thing I can think of which would explain the discrepancy between our experiences. If so, then I might be locked out of the content now, because the caravan avoids me like the plague now, and I don't have an earlier save I can rewind to without losing considerable progress.
How do you even attend the auction/trigger it?
If you lose the fight with the slavers caravan, it's game over, at least for the moment, since the hideout is not implemented yet. What happens if you lose is that they take you to the hideout, and then you get a message that this part was not implemented yet, and then you go to main menu. I assume that in further releases you will have a chance to escape ,if you get caught, but if you already beat them, you probably obtained the map from the caravan slavers themselves, so it doesn't appear in the basement of the slave house anymore. Just a guess. Pretty sure there will be actually something to do at the hideout, in further releases, regardless.
As for triggering the auction, you have to walk inside the slave house, through the front door, and talk to the guy at the entrance. Select the option "Ask to buy" , and then "Wait for the auction" .
Oh, I don't know then, but my guess is that if this is not a bug (and I don't know whether it is one), then maybe it's because you beat the slavers caravan, the slavers don't want to deal with you, so your quest will probably evolve slightly differently. I also think that if this is true, there will be something to do at the hideout, regardless, and it's fortunately implemented in the next release, so we will have to wait and see.