Beat me to it... Don't mess with Tink.Tinkerbell was a very very scary fey, who repeatedly tried to kill Wendy and who helped Peter Pan 'cull' the lost boys when they got too numerous. As J.M. Barrie noted, Tink was too small to hold more than one thought at at time, so when she was angry or jealous she was all anger without check or balance.
It's often less of a lack of concept of "No" than it is a lack of care that someone is saying it.
And on being bound by word; loopholes. If there's something in an agreement that can be exploited, it probably will be.
What are the darker aspects of the fey and the lands of Faerie? Throughout history, and in fables such as Grimm's Fairy Tales, the fey are not very much like Tinkerbell™ at all.
They dominate people, steal/exchange babies, make people dance until they die of exhaustion.
What else are they known for?
What are some good sources of fey stories--gaming books and others?
I don't think calling them 'gullible' does them justice. Their minds are too alien, too different from humans' to grasp certain 'basic' concepts, that every human being takes for granted.A common element in a lot of the folktales I've read iso that the fair folk are very gullible; tell them the pope will be at your wedding, and they'll believe you. In fact, generally fae aren't all that bright- I guess when you have powers you don't need brains.

(Dungeons & Dragons)
Rulebook featuring "high magic" options, including a host of new spells.