my best guess is that the bulk sale option would allocate across the board based on NBV of asset being sold--how does it work?

Not like that.

You group like assets for a bulk sale,. You enter a sale price for that group. So if you have 10 pieces of equipment sold, you do a bulk sale for them with sale price of $XXX.

Goodwill is a separate sale with sale price $YYY.

etc.