Don
Like most things in life, the answer is it depends. When I first started (20+ years ago), I just assigned my contract to an investor who closed the deal. The sellers were fully aware that I would be selling the property to someone else for a profit. I turned in the original contracts and my assignment form to the title company. I was paid by the title company.
But, as I got better at finding and negotiating wholesale deals, the profit margins got bigger and bigger. I found that rehabbers were not willing to pay more than a $5,000 assignment fee so I switched to doing simultaneous closings. I much prefer this method because neither the seller or the buyer knows what my profit is.