People who are not participating in the negotiations do not always come out ahead, and the net social benefit might be negative. Here are some two-party deals that are bad for society (some more extreme than others, but I think they illustrate the need for laws and social rules outside of property protection):
1. You pay me $1200/oz for gold, and I open a gold mine. The gold mine leaks poison into a nearby river, killing tens of thousands of fish, reducing life expectancy in surrounding communities by 5 years, and not being my problem.
2. I am a congressman and you are a pharmaceutical company CEO. You pay me $10 million, directly and in jobs to my underqualified relatives (and me as soon as I retire from Congress), to sponsor a bill which makes competing drugs illegal by extending your intellectual property rights another 5 years. I win, you win, and consumers get screwed with higher prices for a longer time, competing firms go out of business, and the generic version of your drug appearing 5 years later results in 50,000 avoidable deaths among the under-insured.
3. I am a hitman. You pay me to kill your wife, only $20,000, and receive $10 million of her assets that you would have lost in the coming divorce. I win, you win, she loses.
4. I go to a futures market and make bets/investments relying on a 4-degree temperature rise in the next 4 decades, open shipping lanes at the north pole, a dwindling supply of ocean-based food, etc. These are highly leveraged and worth approximately $50 Billion dollars. I then pay you to put CO2 and methane in the atmosphere as quickly as possible, investing $2 Billion in this scheme and improving the likelihood of my payoff by over 10%, making the investment an easy decision. You make tons of money, and so do I. A win for unrestricted capitalism!
These examples are crude, but meant to illustrate certain anti-social impulses inherent in unrestricted deal-making in a capitalist framework. Property rights are not as important as other human rights.