I really think it depends on what their motivation is. I am curious to see why they are willing to sell you a property they have listed with an agent. Personally, I never try to get a property that is listed with an agent and attempt to wholesale that deal. I feel it is bad business all the way around.
Now a homeowner that his in a distressed situation and as their house listed with agent, I may talk to that agent and see if we can come to some sort of mutually beneficial agreement. An example would be, if the homeowner is headed to foreclosure and either they are not going to be able to sell their house before the sale or they have not told the listing agent about the pending foreclosure. In that case I talk with the listing agent (with the homeowners permission) and explain the situation. I let the listing agent know if they release the homeowner and I purchase the house I will use them as the listing agent when I resell the property and they will make more money because the house will be worth more. I also tell them if I wholesale the property then a condition of the purchase is they use the original agent for the resell of the property. If that won’t work then I will pay them a marketing fee. I can usually convince an agent it is better to get some money than no money at all.
This is just an example of what I have done in the past and every situation is different but I can be quite creative also. I also honor all my agreements and that has helped my business and my reputation both personally and professionally.