Anyways im sure that double deposit wont work on mass scale you need an arbiter
Why do you think double deposit wont work?
(I posted the reason for deleting your post in Syscoin thread before I read your question here)
I think it will hinder adoption as vendors will not list things that they have to ensure collateral for AND not knowing if their funds will be locked indefinitely... it doesn't make much business sense although technically it is a superior implementation (however it is less work because no GUI is needed). A vendor would much rather have the piece of mind knowing an arbiter is involved that is impartial and not have to lock up funds, that really should be a discretion on the buyer side that he may have his funds locked until he receives his item(s).
It does open up a can of worms of knowing who to pick as an arbiter, because it has to be a trusted person who has incentive to do good, so you must design around that and create the incentive structure for the system to work. It cannot fully trustlessly on mass scale in this way unless you get an insurance agent involved somehow to provide the collateral for you for a charge.
That all being said, something using CLTV however may work using time locks and extending time locks somehow and I think David knows this.. however it is alot of GUI work to get this right, and more moving pieces, more complications, more unit tests.. but ultimately might be the optimal solution. But at this point 2 of 3 is probably the best way to go.