All assets are priced and traded in markets. All utility can be measured in pecuniary terms. No measurement problems. No crime or war and litigation is does not cost anything. Time is static.
All exchange is voluntary.
The line O Sam to O Jim is the contract curve or efficiency locus showing all Pareto efficient trade allocations. I is initial endowment by Sam and Jim. In this two agent pure exchange economy any point from A to B on the contract curve may be reached depending on bargaining skills. If the number of agents rise they will gradually become price taker and again the efficient trade is uniquely determined. The fat line is the utility possibility curve or welfare frontier.