When a bard learns a spell using the Magical Secrets feature, it counts as a bard spell. Of note, should they replace any spell learned via Magical Secrets at a later level, they can only replace it with one from the bard spell list. Rules designer Jeremy Crawford actually did make a ruling on Twitter regarding this, so this is clear: replacing one’s Magical Secrets spell 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, they can print them off as spell scrolls. Now, who can use that spell scroll?
I see 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 Magical Secrets spell, no matter which spell 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 spell 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 scribing such a spell scroll cannot even use it themselves. Imagine the ignominy with scribing any spell from Magical Secrets: “I cannot read any of what I just wrote down.”
Which interpretation is correct?
(I did search to check whether this was a repeat of a previous question before posting.)