Register Device to Support Profile

I bought my iPad Mini from a seller who has already wiped out/reset everything prior to handing over. No problems for me to set up as a new device, switch on Find My iPad, purchase new apps using my own Apple ID or downloading past purchases.

However, when I tried to register the device to my Apple ID in Support Profile (https://supportprofile.apple.com/) it says that my device is associated to another Apple ID. I have requested the seller to remove the device from his Apple ID but unable to confirm if he has done so. It's been a few days and I'm still unable to add the product to my support profile.

Would appreciate if anyone knows the answer to my queries:-

1) Do I have to wait 90 days before trying again or does that 90 days only apply to associating a device to Apple ID for download/purchase of apps from Appstore?

2) Will there be any problems if the seller does not disassociate the device with his apple id?

3) I was only provided with the box (and original sticker for warranty) with photocopy of receipt and was told that it is sufficient for any warranty claims.

Any replies would be most appreciated as I would like to know if I should cease worrying or start to hunt the seller down... Thank you.