Liability for Flea Market or Garage Sales.

The sad reality is that our society has become so litigious that people are willing to sue anyone for virtually anything, even something as ridiculous as the scenario you've described.  What you've got going for you, thankfully, is what lawyers and judges call the "giggle test."  A lawyer has to ask himself or herself whether he or she could stand in front of a judge and make an argument without giggling.  Personally, I don't think I could make a claim that the reseller of a used board game, particularly one as well known as Monopoly, is liable for injuries sustained by a child who choked on a game piece--at least not with a straight face.  As a practical matter, unless you as the reseller have particularly deep pockets that make it worth the trouble for a lawyer to come after you, any such claim would be against the manufacturer of the game and the retailer that introduced the game into the market.  Besides, most if not all board games with small pieces that present a choking risk are labeled with warnings that put parents on notice that some games may not be suitable for small children or that parental supervision is required.