Only Six in the World, but Cannot Shift it for 12k

It seems collectors for some reason are a bit wary about the possibility of buying the six known Anglo-Saxon helmets from Bab Dadge the dealer even if the price is dropped from an estimate of 35000 green ones to 12000. Could there be some reason for this? Is it the lack of full details upfront about collecting history that would bulk out that cover-all assurance: "legal to buy/sell under U.S. Statute covering cultural patrimony Code 2600, CHAPTER 14"? Or is there some other reason? Seems a bargain, no? And weapons and helmets are very popular in the collecting world, so what is it about this one that is preventing it from being snapped up by a greedy collector interested in such things?

Vignette: a picture of a lime tree from a Polish website