Ok lets play blackmail:
We do a trade and I (Bob Blackmailer) will not release the fund at the end of the trade, so you would loose 1.1 BTC and I only 0.1 BTC. I send you a message that I only will release it if you send me 0.5 BTC.
Would you accept?
Your 10% markup for example is useless, as even just selling yesterday would mean the price moves more than that - anyone who were to send fiat simply would be smart to let the 0.1 BTC be and keep the funds frozen forever just to screw with the BTC seller.
Anyways, you say I sell 1 BTC, we both put 0.1 extra on the table and then you say I need to pay 0.5, to get my 1.1 back? I would for example publish a transaction that depends on the 1.1 that is paid back to me, so there is no chance that this gets cheated... if you want to get your 0.6 (0.1 from the table + 0.5 of my 1.1), then you'd have to release the funds.