Internet Auctions like Ebay
Getting and sending cash is an interesting exercise in trust. My personal experience has been mostly with international purchases of small items where the costs for the seller to cash a check are totally outrageous ($20-30). Especially when the dollar is high, many sellers from outside the USA are happy to accept dollars. Germans in particular like clean, new bills :)
I have also sent cash to a couple of American sellers when I was annoyed by their requirement for a money order for a low dollar item.
I have sent cash less than a dozen times, but never had any problem with it. As a seller, if someone sends cash I think you have to accept it whether or not you said you would. As a buyer, I would not send cash to a low feedback seller, a seller whose Terms of Sale indicate a combative nature, or a seller with a relevant negative feedback history.
You get your money in 3-10 days so from that standpoint it works.