I would imagine that you / the user is not expecting to be surprised at any point if you use a service like this. I would also imagine that the op would fadeout or disable the ability for anyone else to purchase the same gift. However I am not sold on the idea of all the money raised goes to the User on the event.

For example Christmas you could of paid 50% of the cost to 5 items for friends and families hoping someone else would pay the rest, what if they do not? Come Christmas day your money is just passed to these peoples paypal accounts and you have nothing to give them on the day.

A question would delivery be optional to the User or me as the buyer? So I can give that gift to someone and not just deliver it by post?

I do like the idea and if executed correctly a very useful tool for around Christmas time. But it can be hard to predict the response to a service like this.