I'm working with the two conures at the pet store where I work.

is doing GREAT!

He's really excited to see me and is climbing all over my hands and arms. I can get him to step up repeatedly and pass him off to other people. We don't want him getting too attached to me, because I have no intention of buying him (it's really hard! He's such a cool bird!). So he needs to be comfortable being handled by other people.
He's been nibbling my hands and putting his beak around my thumb and holding on or pulling it towards him. It's been very gentle, but today he started biting a little harder than before. How can I keep this behavior from escalating?

is doing less great. Mostly she just wants to bite me. I've offered her millet, sunflower seeds, and dried fruit and she'll dodge past the food to grab some flesh. I've been doing my best to not react, to try to extinguish the behavior, but it HURTS! Recently she started biting and twisting. Today she broke skin. Maybe I'm just getting an extinction burst, but I can't keep going with this training plan, it's not safe.
She bites everyone. There is one girl who can scratch her neck without being bitten, sometimes. She says she rapped on her beak and said "no bite." I am not willing to do this, and don't think it will help anyway. Although admittedly she doesn't bite her nearly as much as I get bitten.
I don't really know what to do. I don't know how to train her if I don't have anything to reward her with.

Here's the Sun Conure on my hand with the GCC in the background!