Assuming that both seller and buyer want to finish the deal as agreed, you can open a ticket (Alt+o) and ask a mediator to push the deal through: state what you want to happen and it will be taken into consideration. The mediator will make an offer that both parties need to accept.
I can’t help but wonder if you are looking at the same trade ID that is linked in the first response. In that deal, the procedure was exactly as you described and then what I described - I am the seller and accepted the mediator’s suggestion, but the buyer has not accepted yet - did buyer not receive the mediator message?
He receive the mediator messages, he could have chosen not to be cooperative.
Just remember that his security deposit is in the line. Its in his best interest to cooperate.
You are experiencing a bug, open a mediation ticket with ctrl+O if that option is not available yet (it is displayed through a button that replaces the trading chat once the trading period ends.