Rodolfo,
Boring as it may be, it is not what I expected or wanted to be done with my lots when I signed up with 'Webinterpret' listing all my BIN in 6 other eBay sites outside the US eBay that I have been on for years. It actually cost me more than I make most months, but I did this to reach collectors all over the world in their language and in their payment currency. It has been successful in that I have gained more new collectors that are also likely to start seeing your lots up on eBay as well. I believe that most of these duplicate listings are grouped together, so the viewer can thumb though these at a pretty fast past. As for taking up space other collector/sellers have up on eBay, do what I do and each week notify their buyers with a link that directed them to only their lots. But, this takes work on the sellers part. I send these links with my message that is going to over a thousand collectors each week. I make my message an important part of Mexican Philately, talking about subject that are interesting and fruitful for their collecting efforts.
Now talk about boring, dealers that list all their items too high priced, long list of the same thing they do not collect or put up penny lots of unimportant stamps everyone has already. I learned quickly when I first started out selling on ebay around 20 years ago. If sellers only sell stuff they do not want, no one else wants them either. If you look at my auction lots every week the stamps vary a great deal. Not all of them are expensive, but all are started out at a fair price and I tell the viewer of all the good qualities and faulty conditions if they have any. I get around 2 or 3 returns a year and mostly due to my mistakes. If the buyer does not get the item in the mail, I almost always either replace it or refund their money. This happens more when I send to Mexico than anywhere else in the world, but I know that Mexico is a good market for Mexican stamps and what I pay out in losses, is covered by the stamps and postal history they receive and love.
So, be a little more understanding of what is happening on eBay and do not judge the sellers as much as the methods eBay has.
Bubba