This list lets the sell create a specific group of people that they are willing to sell to.  The notion is to throw out the bad and only keep the good, the ones that they beleive will pay and leave the slow payers and non payers to someone else. 

However each pre - approved list only applies to that auction and if the seller wishes to do it for all their auctions than they must do it for each auction.  There is no magic button to press and zip it's all done.

Members not on the pre-approved list will get a little box that says that they must be pre - approved by the seller before they can bid.  It is then up to the buyer to contact the seller to get pre - approved or told to go away by the seller.

Buyers and bidders can use the ASK SELLER A QUESTION in the upper right corner of the little box where it can take you to their other auctions. 

To make a pre - approved list for the item(s) you are selling or to block specific members from buying from you or make a buyer block exemption list you can go to http://pages.ebay.com/services/buyandsell/biddermanagement.html

You can also cancel a person's bid if you do not want to go the pre-approved route.  This lets a member bid, but if you've had bad dealings with them before you may just want to cancel their bid - especially if the riff between you two has not yet been mended to everyone satisfaction.  Here is where you go to cancel a bid - offer.com/we/eBayISAPI.dll?CancelBidShow