I've searched multiple dictionaries and Etymonline but the only origin for "flog" that I can find is:
1670s, slang, perhaps a schoolboy shortening of L. flagellare "flagellate."
This clearly relates to its proper meaning, to whip or beat.
However, in (British, and perhaps other) slang, the verb "to flog" has come to mean "to sell" with an implication being that something being flogged is being sold quickly or cheaply.
The meaning is confirmed in several dictionaries, but I am at a loss as to why the meaning has arisen. And so I turn to you.
I've not managed to find any further links between flogging and selling, which has led me to consider this possibility: Is it possible that the two meanings are unrelated? I had made the assumption that the "selling" variant was somehow derived from the same word which means "to whip or beat", but perhaps it's not.
Judging by the demographic from which the word appears to come from (first referenced by authors from around London), and given that its original meaning implied the illicit sale of goods, perhaps "to flog something" (in the sense of selling it) is a form of contrived rhyming slang.
Could anyone back this up?