Trawling throiugh ebay after reading the initial post on this thread, I can buy anything should I choose to do so, so not sure where you are coming from?!
From searching auction sites for so called "top" named brands as far as I can see it's hats off to Young Nails who seem to be the only company that does not distribute it's product (as far as I can tell) through multi wholesalers & appears to have a far tighter grip on where it's branded product is sold (ie not through multiple wholesalers), all down to company profits at the end of the day I guess, as a result it's products it seems do not seem to be easily avaiable to people who are willing to sell to anyone & everyone on auction sites. A close second comes NSI, compared to other 'major' brands considering it's popularity this brand also sems to be less readily avaiable on aution sites.
At the end of the day I think a large proportion of "blame" if you choose to call it that lies at the door of product manufaturers that allow their product to be sold through multi outlets, allowing for more 'leaks'!