I'm a full-time broker and I also work with wholesalers. The wholesalers I work with only put properties under contract that are NOT on the mls. They find the properties on their own by knocking on doors/mailers/bird dogging/etc, put them under contract and then offer the properties to agents like myself, and others, who have potential investor/flipper buyers for these properties.
This transaction works well since the property is not advertised on the MLS and the whole situation is above board and all parties understand it. I then bring my buyer client into the deal and they purchase the property. Whatever the purchase price is that is negotiated, the commission for my services is added on top of that. All parties understand this. The buyer is getting a property that isn't on MLS, isn't advertised, isn't in a bidding war, etc. All parties are happy.
I, and most other agents, won't write low ball offers for properties on the MLS, tie them up and waste the sellers time and the listing agents' time. This is not ethical since you have no intention of really closing. It also gives me a bad reputation in the real estate community if I'm constantly sending low ball offers that won't close - eventually you will get a bad rep and other agents won't work with you - or just disregard your offers as worthless. My reputation is important.
Wholesaling can work as long as it is done ethically and all parties are privy.