Emailing or calling your end-buyers when you get a great deal is probably the best way to flip a deal quick! But using these free ad sites such as the one you mentioned, CL, backpage.com, etc. etc. can bring you buyers. Even if the deal you’re marketing does not work for them you can qualify them, see what type of deals they’re looking for and find it for them.
In the past, I have added quite a few end buyers to my Investors list, but only a couple are active buyers. But the way I see it is: It only takes one Investor to buy the Property!!!