if i need to reactivate/unlock a device(ipad) and there is no way on how to contact the previous owner. how can i do it?.

More information concerning the Activation Lock - and the methods by which it can be turned off:

https://support.apple.com/en-gb/HT201441

Without that information, you can't. Any and all web sites claiming to be able to unlock an iOS device are scams.

Your only recourse is to return the device for a refund. If you can't do that, then it will be forever useless. Have it responsibly recycled rather than just throwing it in the trash.