If Bitcoin continues to go down I was worried there would not be enough money to pay back people who want to sell their Bitcoin. What would happen then.
You mean similar to a bank run. No such scenario can occur. Bitcoin has no fixed price or promised price so the free market will always decide that. Generally exchanges that buy and sell bitcoin for USD will work on the basis of buying BTC from X for a price and selling it to Y fr a price a little higher, that's where they make their profit. If there is less of a willingness from the Y's of the world to buy at a certain price then they will lower this price and in turn lower the price at which they will buy BTC from the X's of the world.
