RAW: Most likely not, however it would be up to the DM.
The Forge Domain cleric's Artisan's Blessing Channel Divinity says (XGtE pg. 19; emphasis mine):
You conduct an hour-long ritual that crafts a nonmagical item that must include some metal: a simple or martial weapon, a suit of armor, ten pieces of ammunition, a set of tools, or another metal object (see chapter 5, "Equipment," in the Player's Handbook for examples of these items). The creation is completed at the end of the hour, coalescing in an unoccupied space of your choice on a surface within 5 feet of you.
The thing you create can be something that is worth no more than 100 gp. As part of this ritual, you must lay out metal, which can include coins, with a value equal to the creation. The metal irretrievably coalesces and transforms into the creation at the ritual's end, magically forming even nonmetal parts of the creation
The wording of the Channel Divinity seems to limit it to specific items from the Equipment Chapter in the Players Handbook. Plate armor has a cost of 1,500 gp, which is far too expensive for the player to make in one casting of the spell.
What is not certain, is if you can make multiple pieces of an item over multiple castings. Since that the feature does not say you can, I would assume that you can't, strictly Rules As Written. However, it would ultimately be up to the DM.
If you wanted to make it easier for the players, you could:
- Allow them to cast Artisan's Blessing 15 times in order to make 15 pieces of the plate armor, each worth 100gp.
- Then, using their smith's tools which they are proficient with, they could forge them manually into a complete plate armor.
They would still be spending the same amount of gold, and it would take 15 days for them to create the plate mail. Especially in Curse of Strahd, where from my experience it was quite difficult to acquire specific items, it would most likely be balanced to allow this.
As for how this may affect the character wanting to sell these items/weapons, you could say that no one would want to buy them as they would be overpriced.
If I wanted to buy some plate armor, it would cost me around 1,500gp depending on what the DM says the NPC sells their plate armor for. Now, it costs the Forge Cleric 1,500gp to make this plate armor, so if they want to make any money they would need to charge more than that.
As the DM, you could say that no one would want to buy items from the Forge Cleric at those much higher prices if you don't want them to.
The Vistani can freely travel out of Barovia, and are them selves merchants/traders. They could easily make, trade, or purchase items for cheaper than what the Forge Cleric would sell to them.
Alternatively, you could say that no NPC would want to risk their lives by purchasing items from the Forge Cleric as Strahd has forbidden them from doing so. These are just a few of many many ways you can prevent the Forge Cleric from trying to set up shop.
