@Jay Hinrichs
You're right, they can certainly search tax records and try to find number. Short of registering the assignment contract and clouding the title there's not much you can do if someone goes behind your back, and an assignment contract isn't going to hold up well in court.
I entirely agree that wholesaling sucks. Its not my primary business model, and in the end I can say really leaves a lot of $$ on the table.
I've actually been on the other side of a tied up wholesale a few times and it's frustrating. Where I wanted to buy a home a wholesaler was trying to sell at way to high a price and hadn't to wait months for it to fall out so I could position seller.
For us (HomeVestors the "we buy Ugly Houses" guys) as a large national brand everything is highly disclosed so I dont think many franchisees run into this scenario of someone going behind our back. We advise our franchisees to initially wholesale for quick cash and then use more profitable exit strategies as they become more comfortable with the business.
In my business model I only wholesale homes that are lower value areas that have no market price support, or if the seller needs a higher price than I can pay cash and I know I can make a decent margin. Otherwise I close.
I should add that I am licensed in a couple states.