Which one is more preferred in the buyers community and why?
I don’t think that either of those is a selling factor. Buyers don’t care about technologies used in a product (to an extent). It’s author’s decision to use libraries that he feels comfortable with and no one should judge.
If it’s a WordPress theme and the buyer is a developer, he would normally create a child theme which he then modifies up to his taste, so again whether the parent theme uses SASS or LESS makes no difference.
I am more of SASS fan as an aside note 
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