Well, subject says everything that I mean — I think it would be useful to get notifications when someone makes offer on trade that you watch. It would help to quickly see if you can beat made offer or it's better to look for another trade.
It's just really hard not to forget to check watched trades every time, so I think that getting these notifications would be nice.
[Notification's subject have to be named "An offer made on watched trade" to avoid mistaking them for offers that made on your own trade]