The operative word in your question is “true” directly placed before the word “value.” You suggest the “true value” of a specific comic book is a few thousand dollars — but all that means is someone might be willing to pay that much for it, based on extrinsic qualities (rarity, for example). To the person running the flea market, the “true value” of the comic book is virtually nothing. There is no “true value” for any object: it’s always a construct, provisionally defined by a capricious market and the locality of the transaction. Things cost what they are being sold for, and they’re worth whatever the seller can get. It’s unethical for a seller to knowingly take advantage of someone’s practical desperation in a crisis (say, selling D batteries for $75 a pack during a citywide blackout), but it’s not unethical for a buyer to accept whatever arbitrary value a seller places on his or her goods.

Look at it like this: Let’s say the person at the flea market was selling that same rare comic for $2,000. You, however, would be willing to pay far more than that; because of its sentimental value and the status it will bring among your comic-book-collecting peers, you’d gladly fork over $5,000. Would you feel the need to inform the seller, “You know, I’d actually pay you $3,000 more than what you’re asking”? I don’t think you would, and no one would expect you to.