He had them signing contracts where they agreed that he would be trying to find an investor for the house, but might not succeed. He had primed them to think that, well, it's like any realtor.
If I go to a realtor and say, "I want to sell my house for $120,000" the realtor can only sell it for $120,000 if someone wants to buy it. Bruce's "You agree it's not my fault if nobody wants to invest in your property" contract meant that in the majority of cases he did nothing he was able to convince the homeowner he did something but just failed. And everyone of course thought they had this binding contract saying it wasn't Bruce's fault.
What got him nailed was that he had been telling people he had successfully saved many homes, but he couldn't actually prove that: cover-your-ass contracts are not actually enforceable to protect the business owner when you can prove intent to defraud, and giving false information about what the business's success rates are is sufficient to prove intent to defraud.