I'm only trying to think of the far future. If we end up having millions of previous trade offers, with an average of 5-6 games per offer that'd mean quite a lot of rows in that table... Meaning potentially slower trade feature, too (although at this point I can either remove trades from over a year prior or something).
I was also suggesting to keep the trade history in a separate table so you don't have to load it every time an user want to look at their tradables (most of the time), but only when he wants to check his history (fairly rarely).
To be crystal clear, I'm just telling this only to defend the solution I proposed, not to push you in a specific direction.
Frankly I really appreciate the work you've done so far, also only you know how things are implemented, so feel free to implement the solution you think it's best (even considering how much more effort you want to put in this project).