Well this is possible only by subscribing, which requires some investments. So better take a sample AD subscription with higher rate of investment per visibility. It means your visibility and leads that you generate while being there will be the key factor for determining the ROI
Every business doesn't have season but many have, so that factor also is important in identifying the ratio of conversion to investment done. There is, in Mumbai SEO Service Company that does in depth research on what could be, on a longer run, better ROI.
So 3rd party advertiser is not in a position satisfy all category, if it has. On the other hand there are 3rd party AD companies which promote single category business area, like real estate or auto mobile or something else. They are successful brands for classified AD in their category for sure. Here you may get better ROI, but there is problem, many times of over AD subscription sales, that means, company takes more AD proportional to the business leads that they tend to get. This area seems to be unseen by many web portals running AD Subscription business. There is a market threshold is any segment of buyers and over sales planning of any service would definitely result in imbalance of business and would affect all buyer and seller.
In every area there is limitation in requirement which needs to be understood very well before getting into it. But in many business categories you will find, due to unskilled or over skilled number, the business opportunity to enhance requirements leads to such imbalance. Though this does seems to be temporary, but who knows what would be the period of this temporary environment is. Many times by the time one collapses, other start budding, which is what is happening in logistics area.