guar·an·tee
ˌɡerənˈtē/
noun
1.
a formal promise or assurance (typically in writing) that certain conditions will be fulfilled, especially that a product will be repaired or replaced if not of a specified quality and durability.
It's true I didn't go to law school, but I'm still not sure what can of worms I opened and why I can't guarantee things.
To be frank, they are questioning what you would do if the price crashed and your buyers backed out and you went bankrupt? If you are making a personal guarantee, that would usually involve putting secured collateral (property/cash) as a financial guarantee to cover potential losses. This guarantee you are speaking of would probably be bound to the corporation/LLC that you are running your business under and would limit your personal liability. If you are not big enough, there is risk to the coin buyers that you will go bankrupt.
The guarantee that Dan made, people found acceptable because they assumed he would rather cover the cost of the losses and continue his business than close shop. I guess they were also assuming Dan is big enough to absorb such a hit and trusting that he would keep his word to make that decision if the time came.