A crack in the cap of a Meisterstück 146 is not a write-off. In 2007 Montblanc's service department in Hamburg charged a flat rate of 49 euros for a new cap; a written estimate can be requested and serves at the same time as evidence to put to the seller. Set against second-hand prices of a good 100 euros at the time for a complete pen, replacement is well worth it.
If the damage was concealed at the time of sale, the buyer has the usual warranty rights against a commercial seller: he can demand that the defect be put right, that is repair or the supply of an undamaged cap, or he can ask for a reduction in the price. With a private seller, by contrast, warranty is usually excluded effectively, provided the defect was not fraudulently concealed.
The right terminology helps as well, since it makes descriptions and complaints unambiguous: a pen has a cap and not a lid, the ring at the lower edge of the cap is the cap ring, and the grip at the front is the section.