You can filter sellers to only include "eBay top rated sellers", however it's kind of pointless. If people pay $20 for an authentic, knowing it's fake, then they'll leave positive feedback if the knock-off jersey was to their $20 satisfaction. Just because a seller has good feedback, doesn't mean that what they're selling is any good.
An example - the first seller I found when I searched "NFL jersey": http://www.ebay.com/sch/jerseychick49/m.html
They have 99.5% feedback and the "top seller" rating, even although all their jerseys are cheap counterfeits.