1 it's only been a few days. They do need time to adjust to their new world, sights, sounds, smells, activity levels etc everything has changed for them. Crabbing is normal for the first week even few weeks until they get adjusted to their new environment and your routine.
2 stop trying to feed them by hand and you will probably stop getting bit. They don't know you yet and aren't sure of your intentions. Treats on a plastic spoon is fine though, that gives them a little space.
3 It doesn't matter if the previous owner handled them or not, your not the same person and they don't know you yet. Just because one person could handle them doesn't mean just anyone else can too. I am the main person that handle my gliders, my boyfriend can pick them up and put them in the cage but that's about it.
I was gifted a joey from my best friend who I knowwwwwww handled him all the time and he was so mellow with her... he was a crabbing nightmare for me for about 6 months before he figured out I wasn't a demon sent to torture him
He ended up being sweet and mellow but it took time.
My gliders are mainly bonded to me, I can't just hand them to a friend without the glider crabbing or jumping back to me. None of my gliders crab UNLESS I hand them to someone they don't know. I am their human and I am part of their colony strangers aren't. Most of this is because I am not social with humans and I really don't have friends over very often so my gliders are only social with me. I had ONE exception to this, one glider named Pico fell in love with one of my human friends who also has gliders, because the glider chose her over me I GAVE her to my friend. There were extenuating circumstances in that case though and I know in my heart it was best for Pico.
Even when breeders do socialize their gliders before selling them the bonding starts at the beginning with the new owners, sure some gliders are super social with their new owners but the majority of the time they aren't, every glider is different as much as every human is different.
Age for bonding doesn't matter either. I've gotten joeys as young as 12 weeks out of pouch and as old as 13 years old some were super easy some were not age had little to do with it. You just have to get to know the glider and they have to get to know you.
Here's the thing, you get out of it what you put into it. If you invest your time and emotion into them they will return it. Yes some gliders just tolerate us but even those gliders bond to us. Some gliders prefer to be in the cage rather than on a human, but you can't take it personal because it isn't personal.
Gliders DON'T hate. However, they can get as frustrated with us as we can with them. If your getting a negative reaction to something you did, YOU have to change how you did what ever it was that made the glider react negatively. If YOU don't adapt and adjust and change how YOU do things the gliders reaction isn't going to change either. Don't expect the gliders to be the only ones that have to adapt to your wants and demands of them it's a two way street.
It sounds to me that you have no problem with taking as long as it takes... that is that attitude you need to hold on to. Don't put expectations or time frames on 'bonding' bonding is no different than building a long term friendship. It never stops evolving even when you think you hit a road block you really haven't. That's when you step back think about where your at and why you aren't moving forward and try something else.