I've seen a few covenants willing to buy from their own, local verditius, despite his Harnessed Magic.
Why? Several reasons.
He'll leave labtexts behind, so the items can be copied more easily. If this is the intention, the charter can easily be fixed to make sure said lab texts belong to the covenant.
Verditii (IME) need more Vis than magi of other houses. This could be a way of providing him with these resources, while still getting something in return. A sort of something of not-quite-nothing deal.
Many covenants have to take what they can get.
The alternative is to waste precious time making these items themselves. "But we can just get new ones in 50 years, right? If we make it that long, we'll have it made by then!" It's called borrowing from the future.
Besides, many other magi could simply create items with expiery, and how would you know? Spend a season analysing them?
Atleast this guy tells you up front.
It would seem to me that while it would not destroy his business it would have a sufficiently negative impact that it could still qualify as a dark secret- after all the real key to a dark secret is the struggle to keep it a secret.
As for familiars, I wonder if they shouldn't also enter twilight, or become magical animals- perhaps beasts of virtue.
Except that according to p. 104, they already are magical, as they have a might score. And according to ROP:M they do not age (or need to eat, sleep, or drink), which means that they would not be dying if their magus they were bound to weren't getting themselves killed.
Fun fact about Verdi lab texts: They almost certainly include the shape and material from their runes. It acts as effective copy protection since only a Verdi can make an item (pre-enchantment) with those shape and material bonuses. Yes, another Verdi can make you a second one with the right runes, but you still need to get one to help.
I would say if they have aging immunity they still have it after being bound to a magus. They will be extremely upset if the magus dies.
Its a reasonable House Rule. Without it familiars become sort of super sages after Eternal Twilight takes their magus. The ultimate in knowledge of various abilities.
Also note that the Verditius could choose to specialize in limited-duration (such as 70 year expiration, or charged item) enchantments. That way, it's not as much of an issue, and you get the enchantment faster! (sort of).
EDIT - in this context, I'd have no problem purchasing an item from the character - if it was (say) a potion that would be used soon after, or that there was a slight discount for the likelihood that 70-year enchanted item might stop working a bit faster that expected.