There are two criteria. One is the 100 sales PA. that you mentioned, the other is a dollar amount. I cant remember what the figure was sorry, but it is a fairly modest amount. ( used to be a LOT more years ago ). Ebay do a revision of your account once per month and if you meet the criteria, you automatically get the badge. If you drop below the threshold at the time of a later monthly revision, the badge drops away. You dont have to apply for it. It just appears on your account next to your seller id at the top of the page.