\Why You Don't Pay Cash For Cars From Strangers

[Kurt S]

> Different wire transfer systems and operators provide a variety of options relative to the immediacy and finality of settlement and the cost, value, and volume of transactions. Central bank wire transfer systems, such as the Federal Reserve's FedWire system in the United States are more likely to be real time gross settlement (RTGS) systems. RTGS systems provide the quickest availability of funds because they provide immediate "real-time" and final "irrevocable" settlement by posting the gross (complete) entry against electronic accounts of the wire transfer system operator.

This has good info on wiring and ACH payments.
http://www.reba.net/news/wtransfer