Trust me. This is hard
Very. Hard.
People will keep dropping out and its hard to manage everything if things go South
Good Luck to you, thats all I can say
Yeah, you're defintely going to need to know that people are 110% set on doing this.
You can't have anyone dropping out at all, because really, it's just unfair on others.
Try to put that in your OP. People will have to know that this is a commitment, because otherwise it will go to stuff with people putting down their names just wanting a present and not being able to send one themselves.
I'd also recommend you tell people to plan ahead of signing up.
It's not gonna be so good if people say they'll do this then either get in trouble or are stopped by Parents.
So, yeah, it's probably best to tell your parents you're spending money on a stranger from the Internet. (Obviously if you're an older member (By which I mean age, not join date, obviously), then you probably don't need to worry about this as much.)
Again with the planning out, you may wish to think about what sort of things you can get first, seeing as its not going to be fully appreciated if you just send a 2$ Gift certificate to a random online store at the last moment. Especially not if someone has spent proper money on you for a Steam game or anything else.