dnd 5e – How do runes work? Can a Rune Knight forge them?

Rules-as-written, runes as described in the Rune Knight’s class feature do not, as you note, create anything that could really be sold. So no, if you want to just like “make a fire rune to sell”, that’s not really an option. However, that said, I always prefer to look for ways to let players do something cool rather than telling them ‘no’, unless there’s a really good reason to refuse them.

This sounds like a great time to flex your DM muscles and let this character use a form of magic item crafting. Pick existing magic items that fit within the conceptual/thematic space of the runes the character knows, enforce all the usual requirements of time and special components, let knowledge of the runes count as the necessary “spellcasting” prerequisite for crafting, and you can either make him hunt down formulas or just declare that he knows some of them based on comparing his level to the rarity of the items in question.

For example, with a Fire rune, he could craft a flame tongue sword, a ring of fire resistance, or other items that are fire-related; with a Cloud rune, he can create items related to weather creation and control, or to illusions. You could also have him inscribing runes on bits of breakable material to create potions and elixirs in an alternate form, if they fit thematically. (I would use this latter as my preferred item if I were carving runes for sale.)

There really shouldn’t be any balance concerns about letting “non-spellcasters” craft magic items. Thematically, a rune knight is clearly connected to physical crafting to harness magical power, if not spellcasting per-se, and the only reason to be concerned about spreading around item-creation capacity would be if you had a lot of PCs trying to all assist with the task in order to reduce the number of days required. You might have to lay down the law at some point to say “your magic is very different from the wizard’s, so you can’t really help each other on crafting projects”, but even that is probably not needed unless it becomes a serious problem.