(I am surprised the item was consigned to an auction house specialising in fine art and luxury items. I thought the vendor would have consigned it to a philatelic auction house.)
Don't be. Sotheby's, Christie's, and the like have a reach unmatched by any philatelic auction house, and anyone--collector, dilettante, or otherwise--remotely interested will hear about a Sotheby's sale. The reverse isn't necessarily true.
Remember, the would-be buyer isn't restricted to a philatelist
with the means to buy it, it's open to anyone
with the means to buy it.