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I believe escrow.com relies on detecting a Whois change which is not infallible.
Can you give a scenario where that method would be fallible? (AND where having the escrow agent take possession of the domain would be an advantage?)
While they DO check WHOIS, the funds aren't released until the buyer affirms that they have taken possession of the domain. Plus, there may be other conditions that have to be met, depending on the specifics of the agreement. (For example, I sold a domain with software source code. The buyer had to affirm that he had received the source code and was satisfied that it was what was called for in the agreement.)
An analogy would be checking shipper tracking numbers, but still requiring the buyer to assert that they received the package in good condition.