That is a very good question. While some classified sites such as Craigslist may highly frown upon people who market websites using a sort of "pre-sell" advertisement on its platform, there are some less known sites who are desperate for an audience as well as advertisers who will, to be blunt, bend over backwards for websites and programs that spout the usual "work at home" this, that, and the next, because they know how profitable the classifieds model can be.
I use them on occasion as an easy way to harvest organic backlinks. They are also extremely good to use if you are promoting a product or service with the website you are advertising. One thing I like to do is suggest interested users to email me so that, one, I grow my list, and, two, they are much easier to sell to as a personal connection is being founded. Some are ethical and legitimate opportunities, products, and services, while others are not, and people are getting more keen at discerning what seems to be a scam and what doesn't, so my advice would be to try and stand out as much away from that crowd as possible in your ad. It does work wonders with the right amount of patience.