Well, think of it this way, an epi pochette new is only $360 new from LV. In order to take the risk of buying on ebay and buying used, people want to save at least $100-$150 dollars, which for small items is a big percent. In general if it was only about saving $50 or something, I would just buy in the store. Now, of course, limited items are a whole different story.
I also think it's because they're pretty tiny. They don't really work for a pochette as well as the mono versions do.