There is no minimum of 400 feedback required to get Top Rated Seller status - you have to be a Power Seller, which requires at least 100 transactions with a total value of US$3000 or more, over any 12 month period, plus some feedback percentage requirements, then for TRS the feedback percentages and low DSR count gets a bit stricter, but according to this: http://pages.ebay.com.au/help/sell/top-rated.html#what there isn't a minimum of 400 feedback required, though it does stipulate that those who have 400+ transactions in a three month period will have their ratings calculated based on that. For lower volume sellers, their ratings are calculated on a 12-month basis.
(What that means - I think - is that evaluations are monthly regardless, but higher volume sellers with 400+ transactions every quarter only have the last 3 months worth of transactions counted in their evaluation).
