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.
In the Dungeon Master's Guide (p.128), it says this about crafting items:
The creation of a magic item is a lengthy, expensive task. To start, a character must have a formula that describes the construction of the item. The character must also be a spellcaster with spell slots and must be able to cast any spells that the item can produce. Moreover, the character must meet a level minimum
determined by the item's rarity, as shown in the Crafting Magic Items table.
A rune knight clearly does not have spell slots, so he doesn't meet the requirements.
However, Xanathar's Guide to Everything revisits magic item crafting and completely removes that requirement. Using the XGE rules, magic item crafting is not limited to spellcasters (except spell scrolls), so you can simply allow the character to craft and sell items of his choice.
According to the XGE, crafting a magic item requires:
- A formula for how to create the item
- An exotic material component
- Proficiency with an appropriate tool or the Arcana skill
- A gold cost
- Time
In this case, you could (if you so choose) waive the need to find a formula for items that fit within the conceptual/thematic space of the runes the character knows and are of an appropriate rarity for the character's level.
For example, with a Fire rune, you might decide 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 as my preferred item, if I were carving runes for sale.
As shown by the XGE rules, there really shouldn't be any balance concerns about letting "non-spellcasters" craft magic items, and even having multiple PCs cooperating to speed up the process is probably not a big deal as long as each of the collaborators has proficiency with an appropriate tool or the Arcana skill. (You may want to put some kind of limit on how many people can reasonably work on a single magic item, though.)
The adventure Storm King's Thunder does have a few items in it that specifically use giant runes such as ild and stein, which grant the holder various powers, and can additionally be used to transfer the rune to a few specific kinds of nonmagical objects (such as a shield or weapon) to grant that item a magical ability. However, the original rune-bearing items are all rare or very rare, so they're probably much too expensive for the purpose of sales, if you even allowed crafting them in the first place; and the item created by transferring the rune is usually weaker than the rune-bearing item was originally.
However, it might be worth looking at the items the transfer can create and using those as the items your rune knight can make without a formula. For example, the opal of the ild rune can transfer its rune to a weapon to make it an uncommon magic weapon that deals an extra 1d6 fire damage when it hits. So you could potentially let your rune knight directly craft a "sword of the ild rune" that is uncommon and has that effect, without going through the rigmarole of making an opal first.
