Lately, I've been buying lots and small collections on
ebay. There are quite a few sellers that will provide a view of the backs with all stamps flipped, 50 stamps or more sometimes. Then I can figure the lot down to the nuts, even fairly cheap stamps, and give the best bid I can. I think I'm bidding somewhere near a high wholesale level most times. I'm winning too often, actually, so there's proportionately more lowball bidders than I remember from years back, fewer bidders overall on many things. With certain countries, particularly tropical ones, I'll just pass on a lot without back scans. The lot then often sells for what I would figure to have paid for an all-faulty lot.
If you as a seller are willing to leave money on the table, that's okay. If you already have the reputation for not providing back scans, your regulars know you so they will bid accordingly. But new bidders will trust you that much less.