A lot of times for me it comes down to wanting to find the cards before accepting.. That said, I'll usually send a message to let the person know..
I find that if I'm getting overwhelmed by things, or as what happened this week, working almost non-stop and feeling sick, it's hard for me to put coherent thoughts out there.
But then again, I'm one of those people who feel I'm being a bad trader when I don't have things mailed off before I have people asking for them in a trade in the first place... But that's on me :P