This thread is the first time I've ever heard of 'flogging' meaning selling something off hurriedly. The predominant way I've seen it used is to refer to administering punishment via whipping.
That is indeed the meaning of the verb "to flog". Like you would to a horse in the 17th century, to make it go faster. "Flogging off" goods to make them move faster is a relatively simple leap (and universal British slang).
