Can a condo unit be sold before a declaration of condominium is filed?

No. A unit is not legally created until the Master Deed or Declaration of Condominium is recorded in the land records establishing the condominium. Therefore a unit deed would be null if recorded before the condominium is created under law.

For example, Unit #10 of the Shadowbrook Condominium would not exist until the Shadowbrook Condominium Master Deed is recorded.
Personally, I would not buy a condominium unless my review of the complete declaration was finished.

Otherwise, you (and I) have zero idea about the community you're (I'm) buying into.

Whether or not it can be sold may be a local legal issue. See your state condominium statute.