Spinster City Apartments

in other words it's gone...and I can't rebuy it....where could it be?

Since Patty and Selma are attached to it, look under Characters in your inventory.

If you sold it, you have to purchase it again. Why cant you rebuy it? Ive sold buildings with characters before and never had a problem rebuilding them.

Sure you sold it and didn't stow it? Like @spyder1952850 suggested, look in your inventory, or store under characters.
Good luck