If I set nSequence to be 0xffffffff and broadcast that transaction.
I think this is a regular transaction cause the nSequence is the UNIT_MAX. It ignores nlocktime
[1] and RBF, so it cannot be double spent by you.
[1]
https://en.bitcoin.it/wiki/NLockTime- Jay -
Thanks. But does it also mean that anybody else won't be able to double spend it even if they have the private key?
Technically, yes. This is if she uses the exact nSequence you used.
The response of the transaction, when you want to double-spend, is determined by the nSeqequence you use and if the nSequence is equal or less than UINT_MAX (0xffffffff) the nLockTime will automatically ignored which means the transaction will not be accepted into any block.