epc is earnings per click, actually earnings per 100 clicks
impressions is a count for every time the banner is shown/seen/displayed.
say every time someone goes to your page that banner is displayed, and you stuff them. well that means if a 1000 people see that banner you will get 1000 clicks and 1000 impressions, so you're telling egay "hey every single time someone sees my cute egay banner they click it, can you believe it, every time, no shit"...sure...
Now if you're controlling how often you stuff people then it's different but it's still a headache don't use it, don't need to, put up a text link to your affiliate if they come check.
I'm stuffing forums for affiliate products that allow forum marketing, and to drive traffic to my site(s).