It has to occur in some exact order over 4 weeks. He is going to sell something, then something else, then something else, then something else. There will be a specific result, there is no way around that. It is completely true that selling voidfangs 4 times in a row is less likely than every other combination added together. It just doesn't matter, because of what I wrote above - at the end of 4 weeks, there will be a specific order. That is why you shouldn't take 4 results of a trial and make a judgement about it.