Actually it means that they sell a consist amount every month on ebay. To be become Power Seller you must sell a minimum of $1000 a month for three consecutive months, then you must consistantly average $1000 a month every month to maintain Power Seller status. You have to keep a 98% positive fb rating and starting July your DSR rating must stay above 4.5. There are more requirements, but those can be found on ebay.
It doesn't mean that all power sellers are honest, but most of them work hard to make PS and maintain it.