When a bard learns a Magic Secrets, it counts as a bard-spell. Of note, should they abandon any Secret-spell at later level, he can only replace it with one from the BARD list. Lord Crawford actually did rule-rulings on this, so this is clear: replacing one’s Magic Secrets is easily done, it is just somewhat unwise to do so.
Say a bard picks Find Familiar as a spell. Massive boon! For dirt cheap he can print them off as scrolls. Now who can use the scroll? Three possibilities:
This is a wizard spell on the wizard list – designed for wizards. Clearly only a wizard can use a wizard spell on the wizard list designed specifically for wizarding ways, right? So obvious: a wizard buying this spell can transcribe it into their book (with a good roll on a good day) – or simply use it and get themselves a familiar.
Any Magic Secrets spell, no matter which list it once came from, counts as a BARD spell for all intents and purposes (i.e. ‘learning, casting and recording’). Should a bard make such a scroll, any other bard can use it. A ‘secret’ no longer! But to be clear, if it IS a ‘bard’ spell, only bards could use this magic item. A wizard would not ever figure it out. A druid would have no chance. A barbarian would accidentally use this scroll as a fire-starter.
You can use any scroll if the spell is on your class list. So almost anyone can use that Charm Person scroll. Thus, the bard writing such a scroll cannot even use it themselves. Imagine the ignominy with scribing any Magic Secret: ‘I cannot read any of what i just wrote down.’
The third one sounds the least fun and least logical, so probably that one. If this is a repeat of a previous question, please allow me to delete this / i did search before posting.