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?.

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.

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

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