[45h34jh53k4j]

Except for the pathological example of Bitfinex (another story), every $1 of coin sold is backed by the $1 that a depositor has made. There cannot be a 'bank run' like you describe unless the exchange is running a fractional fiat reserve. This is not the case is which you describe. If there a massive withdrawal of billions from coinbase, it means that there is a massive deposit of billion TO coinbase to make the trade. In the case of bitfinex, they appear to be doing peer to peer bank transfers to get round their own banking problems. (ie: you want 10m out, i want 10m in, we trade A->B, rather than through the exchange as a middle man).

You simply cannot sell it if there is no buyer to take it. You never 'sell' to coinbase -- this is a front for GDAX, their exchange. Every buy is taken from a seller on their market. The money is exchanged between you and their account, and they take fees.