I traded a house that I placed today after idocs. Both chars below the house sign, the owner started the trade.. nothing happened, no trade window.. so I waited like 40-60 segs and the trade window finally showed.
Tip: while you are waiting for the trade window, do not doble click on the house sign because you will get a msg saying something like "the house is being traded right now" and you will need to logout and login with both chars in order to succesfully start the trade again.