To prevent item crafting spam to get Unique Variants and then transferring them to a better item, you will need to provide an item of the same star level.
I still forsee people transfering craftspam variants to the lower versions of the weapon/armor they want it on then upgrading it/selling it.
I don't really see what you are saying. What item(s) are craftspam? You are paying to craft an item at that star level. All the items still cost Energy, crowns and materials to make. That hasn't changed. What is the problem you are talking about?

A concern you did bring up is, the auction house may take a hit since people won't be so willing to sell UV items since they can transfer it instead. Or it may make UV items more costly since demand may rise on them. There would probably be more items being sold in the Auction House as well due to crafting mania. But, that is why I have the restriction "the star level must be the same".

Even with same stars transfer, it will allow to get pretty good UVs on Boss/token gear wich could be bad (rage craft brandish to upgrade sanrble barb).
You can buy UVs for Boss gear using Unique Variant Tickets. Would it be fair to restrict Boss gear even though you can buy UVs for it?

Also think that different gear has different UVs, so it should be specific for each type (in your example, balckhawk is a gun, blazebrand is a sword)
The game should not allow invalid UV transfers. Like I can't take damage protection from X and put that onto a weapon. All weapons have a set of Variants they can have, same with armors and shields. There are no UVs that would not properly transfer in the same family (with the exception of Bombs and ASI bonuses). My Bonus damage to Fiends can transfer to my Blazebrand since that weapon allows that UV. But I can't take an Attack speed increase bonus and put that on a bomb. Since Bombs don't have an Attack Speed, just a Charge Time.

1* items will be rage crafted, good uved 1* weps get upgraded to 2*, the UV get Transferred to the item u wanna upgrade
my opinion is, this is a bad idea
That's a valid point. Anyone have suggestions on how to help combat this?

"You can buy UVs for Boss gear using Unique Variant Tickets. Would it be fair to restrict Boss gear even though you can buy UVs for it?"
With your method, you are cutting the prices in half since it seems cheaper to transfer.
You would still need the UV to transfer. And getting a UV is not always a 100% chance, and then add the factor of getting the variant you want. It may be a lot of items before you get the UV you want.

Ya, you waste some resources to make an item, but once the UV is removed, it can be sold normally and recover what you spent.
You can sell items with UV so long as you have not equipped it or you have unbound it.