If on signing up for a trade they have contact info You have a way to let them know the trade was cancelled.
If not it's there problem.
Trades are a dime a dozen so don't think to much of it
Try to be nice, and let them know but if it's not possible don't sweat it. Remember that if you're the one signing up for a trade you'll be the one forcing traffic. So if you no longer send you won't get any back, so no harm done