ken kata

The information in the CHUO EXPRESS Excavation page was about Jomon Period items. 

I was hoping I could find more information/websites about any newer excavations that might be the bowls the seller has.

I don't have enough/any Japanese knowledge on key words or phrases to use for the internet searches for newer Mino Bowl excavations.

The ending part of the seller's bowl descriptions had this :

" All four pieces I had TL-tested (Cost about $ 400 per piece) have confirmed an age about 400 yrs =/- 20%.  "

I think he explained they were random from the batch of bowls he had..

This is from one of many TL Testing  websites on the internet: 

Accuracy of TL testing / dating
Generally speaking, when a sample is drilled and there is no information available about the burial environment, one may expect +/- 25% uncertainty, and this is more than adequate for authentication where the question is whether the piece was fired in antiquity or recently. The latest computer technology supports and completes analysis.

Link :

https://artemistestinglab.com/#:~:text=The scientific technique of thermoluminescence (TL) is used,with a faint blue light%2C known as TL.