Lots of bad info here. The fare rules for a V fare generally require an advance purchase. Often 2 or 3 weeks in advance. There can frequently be last minute V bucket availability, but you cannot actually purchase the fare due to advance purchase requirements for the V fare on that flight. There are no shenanigans being pulled here. If there are advance purchase requirements, you cannot buy the fare from any outlet without meeting that requirement. SDC does not require that the fare be available for purchase, only that there is inventory in the bucket. flightstats.com will tell which buckets have availability for a flight. You can't get that info on delta.com.
+1 - DL really isn't that stupid. DL has been tough on SDC for a reason and an agent offering up an SDC without even being asked to waive a requirement is a clear sign that OP's bucket was available, but that a new purchaser would not meet the qualifications. Hence the fare is not offered for sale.