Syag, there are absolutely no “financial grey areas” about this type of financial offer. It is 100% kosher, through and through. The person receiving the offer would have otherwise never engaged in this financial transaction or have otherwise purchased the coins from the Mint. So when he purchases it for $100 and sells it to the person making the offer for $150, he is making an easy $50 that he would have otherwise never had made. And the person paying $150 sells it on the market for $200, and he makes $50. So it is a win-win, with no losers and nothing remotely unkosher.
It is no different than your local grocer buying a half gallon of milk from Golden Flow for $1.25 and selling it to you for $1.75.
As far as the original case being less than market value, there is absolutely nothing illegal, unethical or “grey” in either halacha or secular law for parties to agree to a price that is less than the market value.