If people stop mining it then you can't buy it. Remember, with the current design if hash power suddenly drops enough then the network just stops.
Often what happens after a massive price drop is people stop trading instead of a larger nominal price drop. AKA, you still get some high value beanie baby transactions, but the market is not large enough to really liquidate a collection at even close to the nominal clearing price.
It seems like the harder problem is convincing anyone else to pickup mining again on the other side of your "I own all the coin" transaction block.
So, unless you start over there will be transactions to addresses and you need to be able to correctly sign a transaction to your own address.
PS: Think of it this way, someone mining just one block can't make an arbitrary transaction from any address to any address.
A 5%/day sustained drop is enough to halt transaction processing entirely.
Markets crash discontinuously. It was trading at one hundred, it was trading at one hundred, it was trading at 99, it was trading at 2. Go on GDAX and look at the order book. Lots of demand priced close together at the top of the book. As you go down, the bids are further apart.
As a first-order approximation, that's a decent view into what would happen if a massive bought of selling hit the exchange.
Some people would probably argue it's no longer "Bitcoin" at that point, but at least you'd still hold coins that have some value.