Two-party escrow. What a great idea!
This is much like an old and tested way of doing business on the street. You want some product or service, let's say the price is $20. You tear a $20 bill in half, give half to the provider of the goods or service and keep the other half. Then the bill is of no value to either of you. When the contract is fulfilled you give the provider the other half. That practice works because you cannot profit by screwing anyone; all you will gain is an enemy if you do that, and you will lose the ability to do business again.
Now to do it online requires just a little modification. One reason is that the consequences of making an anonymous enemy on the internet are not significant, it's not like you are going to get cornered one day and get your ass kicked like you do on the street. The other is that you are more likely to come in contact with someone who has a non-financial motivation for screwing someone online, such as a moral objection to some type of business done on the Shadow market. So False Seller creates a listing to sell something for 100 SDT. He puts up 100 SDT of his own funds to escrow. True Buyer sends False Seller 100 SDT and puts 100 SDT into escrow. Nothing is ever delivered. False Seller gets his 100 SDT back from True Buyer, plus he gets his jollies knowing he screwed that immoral True Buyer out of 200 SDT and it cost him only listing and transaction fees.
In order to make this work against such an actor there has to be a way to make the escrow amount significantly higher than the purchase amount. Let's say each party had to put 200 SDT into escrow, it is far less likely someone would be willing to lose 100 SDT to screw someone else out of 300 SDT.
My preference to this is actually we stick to the original mechanics so..
Buyer - puts up 1x deposit + 1x purchase price
Seller - puts up 1x deposit + item
All goes into a multi-sig transaction that has say "30days" to settle after which neither party gets back anything (if either party tries to con the other). Instead the entire multi-sig balance gets spent as a "fee" and the miner of the next blocks get a nice bonus ...
This will ensure that its impractical for the seller or buyer to gain a financial advantage by being dishonest...
Example
Item on sale : GPU @ $300 i.e. 300 SDC
Buyer and Seller agree sale.
Buyer - 300 SDC deposit + 300 SDC purchase price
Seller - 300 SDC deposit + send item
Seller does not send itemafter 30 days (as defined)
Buyer - forfeits his 300 SDC deposit + 300 SDC purchase price = 600 SDC loss
Seller - forfeits his 300 SDC deposit + did not send item = 300 SDC loss
Neither party has gained anything by doing this transaction .. both parties forfeit a minimum of 300SDC and the buyer forfeits 600SDC.
Buyer receives but lies they did notafter 30 days (as defined)
Buyer - forfeits his 300 SDC deposit + received item = 300 SDC loss
Seller - forfeits his 300 SDC deposit + sent item 300 SDC = 600 SDC loss
Neither party has gained anything by doing this transaction either .. both parties forfeit a minimum of 300SDC and the seller forfeits 600SDC.
It would also be nice to have a way for all in the community to know how much money ends up permanently sequestered in escrow, and in what listings. That will give us an idea of the actual money supply, and also of what kind of listings are problematic.
Linking an item sale to a transaction is a big no no - this is a privacy focused solution so why would we leak such data?