This is a technique I use when test marketing, and is the reason why I maintain a few accounts. I will list the same product on each account, then vary shipping, price and a whole range of other variables, then run them against each other for a set period of time, gather the data and optimize it all into a single listing.
I think you are talking more about constant sales, and yes, it is a marketing technique to sell against yourself in certain areas, but there has to be some advantage to you to do it. For example we sell products from Australia to and Australian market at Australian prices.
Then through a second account, we sell the exact same products into Australia from an international location based on their local prices. Through test marketing we have found for our particular range of products there were two distinct markets within eBay.
A price market and a location market!
One market want to buy the product at the best possible price, they don't care about location or shipping, where as the second market feel far more comfortable buying locally, even though the price is higher to do so.
So in essence we are competing against ourselves, and some people may well find that a little strange, but what we are doing is filing a hole in the market to stop a competitor from gaining leverage against us.
The trick is to balance the structure, make sure you are making as much profit from all of them as possible, and that one isn't killing the other account because then it's just a sheer waste of time of money.
Now I'm going out on a limb here as I'm not 100% sure, but I think eBay will allow you to have 8 individual stores dealing in the same merchandise as each other, but you are limited to selling under 20 items that are identical.
Now that's something you would need to confirm if you have a large scale assult in mind.
One thing you really need to make sure you do is not have any bread trails linking your accounts to customers, because it doesn't matter what a customer originally wanted, if they find out that they could have bought that same product for less through another account, they'll let their displeasure be reflected in your feedback!