Please explain the relevance of this document from the 2006 World History AP Exam.Souce: Tomas de Mercado, Spanish scholar, Manual of Deals and Contracts, Seville, 1571.   High prices ruined Spain...

This document is written from the point of view of a Spaniard who would have been thinking about the economic situation of the time in which he was living.  The point of the document is that the Chinese were selling all sorts of goods to the Spanish in the Philippines and were buying nothing from them.

Remember that this question is asking you about the flows of silver during this time -- about their economic and social effects.  This document (and you should compare it to Document 4) is making the point that all of the silver is flowing in to China and none is coming back out.  This was a major issue for the Europeans at this point in history since they believed in mercantilism and the need to have a favorable balance of trade with bullion flowing into your country rather than out of it.

The point of the document, then, is that (from the Spanish point of view) the trade with China was too one-sided, causing the Spanish to lose silver to the Chinese.