The case to be sellers
The Cardinals have plenty of players who would be appealing to contenders. Ryan Helsley, Paul Goldschmidt, JoJo Romero, Andrew Kittredge, Nolan Arenado, Lance Lynn, Kyle Gibson, and even Sonny Gray have all had their names mentioned at least once as candidates. Obviously, not all of these players will be dealt this year, but it's possible 3-5 of them are if the team commits to selling.
The main benefit to being sellers would be prospect replenishment. The Cardinals have a farm system that ranks between 15th and 20th according to most outlets. Only Tink Hence, Victor Scott II, and Thomas Saggese are on the top-100 list according to MLB.com, and Hence is the only one in the top 50. Dealing any of the aforementioned players, especially Ryan Helsley and Sonny Gray, would net multiple top-100 prospects in baseball in return.
Being sellers will allow the team to get a return for players who are on expiring contracts like Paul Goldschmidt, Lance Lynn, Kyle Gibson, and Andrew Kittredge. Rather than losing these players and getting nothing in return during the offseason, the Cardinals can at least garner a minor leaguer with some upside as reimbursement.
Being sellers would excite some fans in a sense. Young prospects like Tink Hence, Sem Robberse, Victor Scott II, and even Quinn Matthews could see greater exposure. The team could lean heavily into a youth movement by providing time for their young position players to shine.
The issue with selling, however, is that fan interest plummets. The team is no longer outwardly competitive, and droves of fans will continue to not show up. Seeing the top level of a baseball franchise put effort forth bleeds down to even the lowly fan. Player morale will also drop, and the work that John Mozeliak did to revamp the clubhouse this offseason gets reset.