This is actually quite difficult to fix.
Reasons:
1. The value of items in the game is much too high.
2. Javelins do too much damage.
3. The base value of a weapon is determined in large part by how much damage it does.
If you let players craft powerful weapons, they will always be worth too much money. The only way to fix this would be to reduce the overall value of items in the game and reduce the amount of damage the most powerful weapons do. I highly doubt Taleworlds will do either, making this problem unfixable.
There is a mod that reduces the value of items in the game and that mod will reduce the value of crafted items. This is the only fix that I am aware of.
That is most definitely not the
only way to fix it.
For starters, there is no reason why the main determinant of the value of a weapon
has to be how much damage it does. It
should be mostly base material and smithing difficulty, as well as quality of the work itself. If it's cheap and easy to make it shouldn't be worth much, no matter how effective it is in battle. If it's difficult to make and uses expensive materials and is actually made flawlessly, it should be worth a bunch, but you also also spent a lot of money on the materials and a lot of time on skill training.
There are already quality prefixes like "rusty", "cracked", "splintered" and so on, that (I assume) can be attached to an item as it is generated and affect its sale value. So it genuinely should not be a big implementation issue to add more, such as "shoddy", "crude", "subpar", "flawed", "botched", and attach them to items that receive quality penalties in the crafting process. Give them resale value penalties of up to 95% or something.
There are many, many ways you can rework Smithing to be something better than it currently is (including how the skill is trained), especially when you don't have the restrictions on what you can do that modders have. I suspect that's also part of why it hasn't been done yet. When you have many options, it takes longer to figure out the best one. And because Smithing is tied into the general game economy system, it's difficult to re-work Smithing without also re-working the economy, so it makes sense to leave it until later in the Early Access schedule.
In the meantime - single player games don't need to be balanced to be fun. My chosen workaround is to never sell anything I've crafted. I either use it or smelt it.