I suggested something similar (way later) here:
https://bitcointalk.org/index.php?topic=196136In fact, I think a reasonable solution could be implemented by adding a *single* instruction to the script language: suppose you have "OP_CHECKSIG_VAL val sig pub_key" which checks if sig is a valid signature of val under pub_key. Then you can create e.g. two transactions as follows:
- TX1: Take 1 BTC from TX0out, give 0.2 BTC to Addr1, 0.8 BTC to Addr2
- TX2: If you find a double spend of TX0out, give 0.2 from TX3Out to Addr1 and a fee of 10BTC to the miner
The buyer gives both transactions to a vendor who owns Addr1 who additionally checks that TX3Out really has 10.2 bitcoins and is still open. If at some point the vendor gets a double spend he can plug in the values into TX2 so that it becomes valid. Miners will be happy to mine a transaction with a 10BTC fee.
The suggestion in
https://bitcointalk.org/index.php?topic=196136 is somewhat nicer because it increases the chance the vendor gets his money, but more complicated.
Interesting, but as you say it is more complicated. I like to keep things as simple as possible. Also I doubt it will always be possible to determine the vendor. Without an incentive of getting back their double spent coin only very few people will attempt it so it is not necessary to pay the vendor.