But if you only sell direct, and the product appears on Amazon at a lower price, it is blindingly obvious that the product on Amazon must either be counterfeit or stolen.
Even assuming the manufacturer is refusing to negotiate lower prices with resellers..
The first sale doctrine protects your right to sell items that you've purchased... even if you want to price it at a loss.
Amazon, for example, sold ebooks at a loss for years. Obviously you wouldn't say Amazon's ebooks were counterfeit or stolen.
And Amazon does not have sole ownership of that tactic. Others have an equal right to sell items at a loss, even against the manufacturers wishes.
Other examples of when this occurs: going out of business sales; end of season liquidations; etc. and various reasons to liquidate inventory: bad buys; changing product categories; etc.
That doesn't apply when someone has a large quantity, though. Many of those sellers have thousands of feedback, unless they have a large number of friends, it's not from gifts.