Is it safe to publicly display user IDs? (The primary key of a user from the database)

I currently have a website where all users' IDs (the primary keys in the 'users' table) is displayed on their profile. The reasons are:

1) Username is not unique (don't want it to be unique)

2) Users can send each other messages and gifts and money by using their user ID

3) Any user can send any user money, there's no other way (i can think of) which will otherwise guarantee that the money is going into that particular user's account. Email/cell number/username are not unique enough in this case.

4) The user's id will be an integral part in searching for the payment reference.

It did not occur to me before until I watched a video tutorial on Cookies (unrelated to the question above) where he says "cookies should not be used to store information that hackers will want, like a user ID". Which intrigued me to ask this question. My whole website project is based on the user ID public to other users. Silly, silly me!

Where can I find information on this i.e. if hackers can really use the user-ID to do bad things?