Unless the seller is actually DirectAdmin (John / Mark) the legitimacy of the auction im assuming would depend on the type of license it was sold as.
If it was sold directly as a lifetime license with a genuine value of $299 USD i would assume its perfectly legal.
If the license was sold through DirectAdmin as a lifetime license under the internal pricing structure (with an approved internal partner) i would assume its against the DirectAdmin agreement.
As stated on the pricing page; 'Internal licenses must always be sold with a server (reselling the licenses by themselves is not permitted)'
From the look of things, it is either John or Mark who have placed the offer, or it is likely to be an internal license sold against the agreement set by DirectAdmin... To be honest i cant really see a $299 license being sold for $99....
The only one who can confirm this is legal / illegal are the DirectAdmin staff and im sure either way they will do what they need.
Chris