@legendaryswordsman I was pretending he helped them capture a couple more off-screen, but some comments in this thread makes me consider that if the craftsman could then breed more (inside a trap, so they can't escape and he can catch them himself) then this would have been a vital help by them for him to get the "seed" (assuming he lost his previous one to age or shenanigans or something) in which case it'd also be ok.
Just as long as they didn't help him make a single one and he can't make any more, then giving away one of them for free is fine.
But if they helped him make a single one and he can't make any more, then it's fine they get to buy it, but they shouldn't get it for free..