If the iPhone had a passcode on it, all data will be encrypted, and without the correct decryption key, all data any forensic tool would be able to get out of it, would be useless, unreadable, scrambled nonsense. That said, the decryption key could theoretically be brute-forced, but if the device is on a recent version of iOS that brute-forcing process would not exactly be an easy thing to do either. They use quite long keys and have limits to how often you can attempt with a new key. - Furthermore, if you do the restore, new data will be written to some of the blocks meaning not all data will be intact so even if it were brute-forced there may not really be anything left.
In essence, unless you're hiding nuclear codes on it, and the buyer is the North Korean government, I think you'll be fine.
