I have barter at 100 which makes most merchants buy from me at 90% value and sell to me at 110% value.

However Chet in Goodsprings, Nash in Primm and the Dino Shop guy in Novac have all, for different reasons, agreed to give me a discount.
That means all 3 of them buy and sell at 100%

Are there other merchants who will improve their rates in a similar way?

Update. I have found a list on a website that lists 6 other merchants and points out that faction reputation changes things too. So I think this question is answered now thank you.