I am a little unclear on how epn works. Do I need to set up a storeish website to use it or can I just link to the sign up page? Also how does it determine payouts? What if I refer someone who is already signed up to ebay? Any help would be great.
No you don't need a store, and yes, you can just link to eBay's sign up page. Or you can use their widgets, banners, and editor kit stuff. If you don't want to pay for a shop script like BANS etc but still want to make a store, you can try something like TWP Auctions (my free script) or mcjiffy.com - or if you know a bit of PHP you can code one yourself using the free RSS feed or ePN API. Basically, if you get an eBay user to make a winning bid or Buy It Now, you'll get a commission. If you get someone new to signup with eBay and make a winning bid or BIN, you'll get an "ACRU" - which is $25 (although this is changing soon). More info on their payment structure here.