First of all, only do this with trusted players, as you can get scammed from them.
If you have the kiosk, place 100 credits in the first prize slot. Then you can create a "quest" for them to do. It can be anything you want, such as saying something that only the two of you know before hand, or having them do something. I'd just do one task. Then you do the trade (at least I do that first if I'm giving the credits). After that, have them do the "quest".