sorry, but I am probably not the right agent for this work."
no "probably" about it. If you don't want to represent my book, then you're DEFINITELY not the right agent for the work."
I agree. "Probably" doesn't soften the rejection, it just weakens it. Either you ARE or you AREN'T the right agent for the work. There's no need to make yourself look wimpy or indecisive in order to not hurt a writer's feelings. It's a REJECTION after all--the writer isn't going to be happy even if you say "probably."