Thanks Conder. My basic 1799 approach is to test to see if it isn't not a 1799 (does that make sense?). Then as most mistaken 1799s are 1798s, I see if it matches a 1798.
I did some photo overlays. The left edge of the I was the only obverse letter strong enough to identify. It did not line up with where the I should be on either 188 or 189. However, the bow, face and hair as well as the upper circle of the last number of the date matched 189 outstandingly well. The reverse, with the exception of the 1 in the fraction also was a great match. I can see why the seller and three dealers he took it to, said it was genuine.
I was able to align the I to about 3 or 4 1798 varieties. The berries on the reverse matched up on just two. The 1 and the die clash did match on 174 only.
Anyway, I've informed the seller. Oddly, when I offered to identify the coin, the seller was enthusiastic and communicative. Now they aren't. Guess I'll need to contact Judith at
ebay.