How much traffic did you have when you started selling ads? What type of CPM are you able to see?
The reason I ask is that I run a niche community site that gets a decent amount of pageviews but I'm not sure how much I could make by selling ads against it.
I've interviewed maybe half a dozen sales people and worked with 2-3 who didn't work out. I have no advice other than to say it's really hard to find someone good. A great ad sales person is someone who wants to make a lot of money. Those people typically steer clear of early stage ventures because there might not be a lot of money to be made. That leaves people who are ok with not making a lot of money. If you hire those people, you won't make a lot of money.
I started selling ads on day one. It's always possible to find someone at some price who will want to advertise on your web site. We see extraordinarily high CPMs on our site because we work very hard to sell them on the quality and uniqueness of our audience. Basic salesmanship stuff.
Do you mind sharing how much commission you pay your ad sales person?