You should have actually read the entire definition
"The Northern form boughten (as in store boughten) features the participial ending –en, added to bought, the participial form, probably by analogy with more common participial adjectives such as frozen."
You can use boughten as a participial adjective, just like you can with frozen, as a modifier to describe something. And because a participle is an adjective form of a verb, it's not a verb anymore and cannot be used as such. I keep my throne, and to the dungeons with you.