I live in a fairly rural area but we have a 4 town joint facebook group for buying, selling, and trading items-similar to a widespread online yard sale or a local craigslist. When buying items you meet up in personal in public places and trade the cash for the item.

Tonight a woman, let's call her Sherry, hosted a sale post for a figurine from a limited edition, no longer purchasable McFarlane collection. It appears to be in very good condition in the photos and could easily be resold for upwards of 150$ or more. She then hosted ANOTHER one from the same set, and both figures she's only asking 20$ each. 

I requested to buy the first figure, and to be honest I don't plan to resell it. I want it for my house. That being said, the lady was very clearly unaware of what she has and doesn't even know the name of the figurine since it is no longer in the box. Although it was her choice to price it as she did and not do further research, I feel a bit guilty and almost like I am exploiting her lack of knowledge of the item. 

Should I offer her a little more money for it? Should I tell her what it's really worth? Or should I just take that she posted it for 20$ and pay exactly that? I'm probably really overthinking it, but I know if it was me making a sale on an item and accidentally priced it way too low I'd be very upset later. 

TL;DR: A woman is offering to sell me a ~150+$ figurine from McFarlane co. for 20$, WIBTA for not telling her it's true worth?