In step 5 of the procedure described in https://en.bitcoin.it/wiki/Smart_Property#Theory, the buyer creates a partially signed transaction. Will the seller be able to change an output prior to signing and broadcasting - and thus returning the token to an output he may redeem himself?
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In one word: no. Changing tx output after signing makes existing signature invalid.
( There are some special cases, like signature hash type SIGHASH_SINGLE or SIGHASH_NONE, but we assume that the buyer uses correct signing SIGHASH_ALL )
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There's nothing more correct about SIGHASH_ALL than other signature types. – Matthieu Apr 1 '15 at 18:13
