Yes, it's usually plural, but if this is a joke, then the singular is used for comic effect and he will be talking about a particular asset, which no doubt would be known to us if we could see the entire joke. To repeat what I said about "death-proof", jokes or amusing phrases are often made by slightly modifying a commonly used phrase in an unusual context.
Speaking of context, questions like this really need a bit more!
In this case it is comical. To liquidate ones assets means to convert ones "worth" into cash. In this case he is literally converting his posessions into liquid not money.
He is "Liquidating" in the literal sense of the verb.