I decided to make this because there are a few things that can make you get a trade lock but I have not seen anyone post.I got one today from a very specific form of actions which I am sure many might miss so I will post what I did.

1. Trade through steam client interface
   
2. After trading list an item on the steam market using the steam client interface
   
3. Intiate a trade offer via steam website (when logged into the website trade offer urls often open the websites trade window)
   
4. Doing these 3 things in that order got me a 7 day lockout

Never had this happen to me before, but then again I never did this exact order of actions either.

Update: I dunno why, but I tried logging out of the steam website a few minute ago and now I can trade again. Not sure what to make of this other then possibly the above actions trigger the 7 day trade on a fluke.