If your treasury goes below -2k it auto defaults back to -2k at the begining of the following turn. In other words, say you have 1BC, but something that pushes your treasury to -18k, the next turn you jump all the way back up to -2k regardless of how much money you are making.
I cannot find any evidence of things being sold off, or of the action that cost the money not working.
Also you can upgrade ships witha neg treasury.
Basicly this means you can buy any wonder, any amount of upgrades, etc as long as you don't mind going to -2000bc, which takes 2 or 3 rounds to earn on the realy big maps.
