If she was indoor it seems like she wasn't held much a baby. That said some ferrets are nippy though, i have one Moonshine who is alittle nippy she will only have an odd nip though. I dont know what to suggest really you could try picking her up when she comes to you and remind her just say no biting, Moonshine understands this now.

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Hi there
I would say she is just doing what she can get away with! The fact that she is picking and choosing when she does it just seems like she isnt totally scared although it may be a factor.
I would suggest the same as dkdream and say handle her little and often.
If she bites when you are holding her dont put her down when she bites you, keep hold of her until she is 'nice' again and then put her down. Putting her down because she bites will make her think that that is how she lets you know she's had enough.
Keep plenty of treats to hand and make sure she is always fed before you handle her. Also dont put your fingers or flat hands near her mouth, instead make a fist so she actually CANT bite you. If she goes to bite your fist you can say no and she wont actually hurt you. Reward any positive behaviour with a treat or a lick of vitamin paste.
I am by no means no expert but am currently dealing with a foster ferret who was a pretty bad biter (latch on wouldnt let go!!) and "seem" to have made ok progress with her just by doing things like this.
It will take time but if shes young she will probably come round quicker than an older ferret. My biter is prob 2-3yrs old and I actually think she has got into the habit of biting and even now 4 mths later will still try and bite however she will let go when you say "no" now - shes just a grumpy bu**er!!

Just seen you said you scruff her - some times scruffing doesnt help, I have one who will bite more if you scruff him. Try some of the above without the scruffing.

I'll try all the advice I've been given, something has to work!! lol
Thanks for all your replies