The primary purpose of the passage is to propose:
a way to curb illegal digging while benefiting the archaeological profession.
The first paragraph identifies two major problems faced by the archaeological profession: inadequate funding and illegal digging. The passage indicates that the author is going to suggest how to remedy both problems, thereby benefiting the archaeological profession. The author proceeds to propose allowing the sale of excavated artifacts and to explain how this would solve both the problems. The author then supports the proposal by countering possible objections to it, and in the last paragraph explains how the proposal would curb illegal digging.
Thus, the way information is organized in the passage indicates that the author’s purpose is to suggest that allowing the sale of excavated artifacts would provide funds for the archaeological profession and curb illegal digging.