1. A system for managing collateral allocation and substitution in general collateral repurchase agreements, comprising: a plurality of trader terminals; an intermediary computer system adapted to: communicate with each trader terminal via one or more communication links; facilitate agreements between buyers and sellers for sale of collateral; receive allocation instructions for an agreement from a seller trading terminal; store information regarding collateral that has been allocated and mark said collateral as allocated; receive substitution instructions from a seller trading terminal to substitute allocated collateral, wherein: if there is one buyer who is allocated the exact amount of collateral that the seller wishes to substitute, that buyer is substituted; otherwise, if there is one buyer who is allocated a higher amount of the collateral that the seller wishes to substitute, and that amount is sufficient for the substitution, that one buyer is substituted; and otherwise, buyers have their collateral substituted on the basis of a priority determination.