Basic
1. Make sure the place you're buying from has their Entrecard near the top of their page, so users don't have to scroll down to see it, especially very far down to see your ad.
2. Judge their traffic: Do a quick scan, see how many comments they're getting, if they have a page counter on their site, or even better; see how many subscribers they have if they have a FeedBurner subscriber image.
Advanced
1. Judge their traffic even more: Go to Alexa.com and type in their domain name, see how many visitors they get, or go to pr.blogflux.com and chexed out what their Google Page rank. Don't rely on those completely, sometimes newer blogs with traffic wont show up in Alexa or with Google Page Rank
2. If someone doesn't have their EntreCard near the top of their page, it may still be worth the EntreCredits if they get a lot of traffic.
