I have been trying to find this collection for a while now; this is the first forum i've asked in.
It was probably a short book, with stories in the style of Christopher Pike and R.L. Stine. I only remember two of the stories.
1.) A girl goes out with her parents--camping or shopping or something--and meets a strange girl around her age who has a t-shirt with two lips on it. She becomes friends with the girl and stuff happens (sorry, i don't remember!), and in the end, the girl has somehow taken her place in her home, and she looks down at herself and sees she is wearing the t-shirt with lips on it, and has to wander alone now.
2.) A boy runs away from home because he's mad at his mom, and is taken up by a nice motherly lady, who tricks him into entering her house, then locks him in her basement, where he is alone in the dark, above a trapdoor. In the trapdoor is something that eats him, as the ending is implied.
Thanks for any help you can give! It's weird, but I want so much to remember these!