Windjam

you sold the call. you earned your premium.
you are going to renege on the deal and not deliver the contract. (which means prices of the underlying has actually gone up - which is why the buyer has exercised the right to use the option).

not sure about market risk and/or liquidity risk in this situation. we are talking about the call OPTION not the underlying itself. There is however a credit risk to the long party (which is not unfounded since you are, as the short, going to renege on fulfilling the obligation). Short call thus becomes a liability as well.

CP