Yeah. That's why I'm telling you that it's hard to sell a site.
But if your job is buying and selling forums or blogs, then you can still love your job and make money. You don't have to hold onto a blog long term. Think about how many people start a blog, love it and then abandon it due to time constraints. Might as well sell while the blog (or forum) is still popular and you can earn money off it.
I had my forum up a good long while (maybe a couple months) before any ads went on and did notice a considerable Adsense earnings increase when the ads went up. More recently, i retooled and now ads appear to guests only right below the thread starter and I think it really made a difference with adsense earnings.