Before you consider buying an ad slot on a website its best to do a little bit of research before commiting. For example how long has the site been running? Are there frequent updates? Do the visitors leave feedback where applicable??
Just taking a bit of time to learn how long the site has been around for and whether its active can save you a failed investment.
Its not uncommon for webmasters to go off the boil a bit once a website doesnt take off the way they planned. Their updates can reduce in frequency resulting in the community fading away. And what is the point of serving your ad to a thin audience??
Try to take the time beforehand and not dive in.