I see nothing wrong with negotiating after secession. The South was willing to let the military equipment and supplies be removed by the North.
How about negotiating after seizure of the property? The south seized armories, mints, customs houses, military facilities throughout the south without compensation. Then, once they had posession, we are to believe that they would have paid a fair price for them. If one party takes the property belonging to the other party, that party is at a distinct disadvantage in the negotiations. It no longer has posession of what belongs to it, and has to take whatever the first party offers no matter how unfair. Or else fight for what is theirs. That's not negotiation, that's brinksmanship.
I can just imagine the North sending some kind of half-assed 19th century Hans Blix down to Dixie to search for weapons. [snicker]