That's a question to be answered by the seller with high resolution images.
The higher value (potential) of the item, the better the description and images you should expect.
Of course everyone is trying to find a $2K book for $20, and don't wish to publish the info.

To me, "Rear board with very modest dust stain", means the board is either so dusty that it can't be carefully "dusted", or it's moisture and dirt related.