Agreed that I would NOT do that. First of all, those weren't "wet look" cones when new, so it isn't restoring them to newness. Secondly, the coating WILL have some effect on sonic performance. A very minimal thing, like painting with liquid ink that weighs almost nothing, would be acceptable, but a coating really isn't, unless you are deliberately trying to change, not restore, the speakers.
The ones in that auction look poor. Sad to see that done to nice little ARs, when it could have been done right. The sloppy way he smudged the edges of the back with the stain illustrates the "amateur" job done on them, confirmed by the black stuff smeared ALL OVER the speakers. As Maxseven said, the frames and basket should be matte -- usually best just cleaned, not painted, but painting a basket is okay if the original was painted.
As noted, most of the AR fronts were pretty ugly, probably because they were never meant to be seen. I covered mine with a thin layer of felt, both to make them look .good and also to improve the acoustics. Looks great, sounds good, and it's also fully reversible (the felt is held on with thin double-sided tape, and it can all be removed to show the original front).
