I think the DAO should absolutely stay away from building a secondary sales market place. In my opinion, the further away from secondary sales ENS is from sales the better. Take a look at what happened with the OpenSea employee. I would not be surprised if the same enforcing authorities are watching people here.
Hypothetically, if ENS DAO created a marketplace and launched it today, I would give it less than a week before someone publicly accuses members or any affiliates with insider trading, collusion, synthetic secondary sale prices etc.
Although I have not observed any one person(s) or read conversations of such events happening, a separation between the root and the branch need be as far away as possible to prevent such erroneous accusations.
ENS is open-source and operates to facilitate a digital good for individuals and other entities to establish a form of identity on web 3 decentralized platforms. ENS DAO does not aim to facilitate profits from secondary sales of identity assets and only receives income from the registering of names that do not have a registrant address set. The cost of registering is fair and the tiers of price for registering applies to anyone attempting to register and the name being registered is available for anyone if there is not a registrant address in the meta.
However, an individual may trade their ability to access and utilize a registered ENS name privately utilizing a third party service that provides the agreeing parties; the maker and taker a fair way to exchange a digital good safely. It is the responsibility of each party to understand what they are receiving in return for their offer and vice versa.