I take some of these numbers reported with a grain of salt:
The cheapest any $20 plaque has sold for on eBay has been $50.00 each, including that there was a lot of 10 of them that sold for $560
The $100 hasn't sold for less than $110. The $500 had a best offer accepted so no exact sale info, but the asking was $400.
If some guy showed up and offered me a mix of all denominations for $40 to $50 each...
First, are they genuine? Are they damaged? If no red flags, send him over to me so I can buy 10 or 12 of the $500 for $40 or $50 each, along with the rest of the 20's 50's 100's 500's and 1,000's! Wouldn't matter that I'd only make $10 or less or nothing on the $20's, the blended price is too good to be true. That you're offered the entire hoard of them so you could control distribution and not flood the market with them...
Something doesn't add up here... Just saying.