How did the slang meaning of "flog" come about?

The OED says

c. slang (orig. Mil.). To sell or offer for sale, orig. illicitly.

with examples from 1919; but it doesn't give a reason for that meaning.

This use of to flog sth in the sense of to sell sth cheap to get rid of it reminds me of German verkloppen (North German). But the consonants are in different order. German has klp, English has fkg. If we assume that f belongs to the series p b f v and g to k g ... h we have the same consonants in different order.

I have found quite a number of such words, about 250, with the same meaning and corresponding consonants where the arrangement of the consonant is different.

So I may assume that to flog and kloppen (in verkloppen) belong together. Such things are not according to Grimm's findings, but they exist nevertheless.

Just one example: to slump corresponds to German plumpsen.

Eric Partridge, A Dictionary of Slang and Unconventional English, fifth edition (1961) has this entry for flog:

flog. To whip: from ca. 1670. Until ca. 1750, c[ant]; in C19–20, S[tandard] E[nglish]. [Elijah] Coles, [Dictionary] 1676. Prob. an echoic perversion of L. flagellare.—2. To beat, excel: ca. 1840–1910.—3. In late C. 19–20 military, to sell illicitly, esp. Army stores; and in post G.W. c., to sell 'swag' to others than receivers. F[raser] & Gibbons [Soldier and Sailor Words and Phrases (1925)]; B. & P. Ex flog the clock ["move its hands forward"] or flog the glass ["turn the watch-glass"]. [Cross reference omitted.] —4. Hence, to get the better of (a person) esp. in a bargain: military: 1915. F[raser] & Gibbons.—5. Hence (?), to exchange or barter: from ca. 1920. Anon. Dartmoor from Within, 1932.—6. See flog it [the entry for which reads "To walk: military: from ca. 1912. F[raser] & Gibbons. Ex the effort {flog oneself along}."—7. (Ex [sense] 3.) 'To offer for sale (especially when financially embarrassed ...),' H[unt] & P[ringle, Service Slang (1943)]: Services, since ca. 1935.—8. To borrow without permission: Services: since ca. 1937 H[unt] & P[ringle]. (Cf. sense 3.)

Partridge does not persuasively explain how English made the jump from flog sense 2 ("To beat, excel") to flog sense 3 ("to sell illicitly")—unless you find his deriving the usage from "flog the clock" and "flog the glass" persuasive. But Partridge seems quite confident that sense 3 emerged in the late nineteenth century and had a military origin; and the emergence (in the services) of flog in the sense of "to offer for sale [under financial pressure]" suggests some underlying institutional memory of flogging in the sense of selling illicitly, from several decades earlier.

John Ayto & John Simpson, The Oxford Dictionary of Modern Slang (1992) has a much shorter entry for flog:

flog verb 1 trans Brit. orig military To sell. 1919–. M Drabble Let's go ... and look at the ghastly thing that Martin flogged us. (1967). 2 intr. and refl To proceed by violent or painful effort. 1925–. Times [Lorry drivers] are being encouraged to 'flog on' even in bad weather (1964).

Tony Thorne, The Dictionary of Contemporary Slang (1990) sems to confirm Partridge's earlier genealogy of the term:

flog vb to sell. A common colloquialism in Britain which would still be cosidered slang by some speakers. The word originally referred to selling off military stores illicitly and is said to derive from a 19th-century expression to 'flog the clock', meaning to put the clock forward to shorten the working day, later extended to other devious behaviour.

2019 - the word flog has made it into Australian slang - quoting the Age newspaper article by Matilda Boseley from June 8, 2019: A Carlton supporter has been evicted from Marvel Stadium during Saturday's Blues vs Lions game for reportedly shouting abuse at an AFL umpire. 3AW has reported that the man was evicted from the stadium during half-time for calling umpire Mathew Nicholls a "bald-headed flog", as he ran off the field.

The term has been attached to a particular player of AFL named Adam Goodes and some claim it is a disguised way of racially vilifying him, others claim the word has been used well before the word [meme] became associated with Goodes and has been used in many sporting situations to more politely describe a person who is considered a "wanker" or a "shit bloke" both terms that are common in Australian language as this article from Buzz feed explains

Without much evidence, I suggest that to flog = to whip = to urge along (as in the "dead horse" analogy). And thus the man flogging the iffy goods, was urging along the sale of the iffy goods: he was encouraging the sale of his own items.

For what it's worth, here is another "business" word relating to informal sales:

verb swaps, swapping, swapped [with object]

1 Take part in an exchange of. ‘We swapped phone numbers’

1.1 Give (one thing) and receive something else in exchange. ‘Swap one of your sandwiches for a cheese and pickle?’

1.2 Substitute (one thing) for another. ‘I swapped my busy life in London for a peaceful village retreat’ (LEXICO)

It previously meant "to strike", which is its only other relation to "flog":

swap (v.) c. 1200, "to strike, strike the hands together," of uncertain origin, possibly imitative of the sound of hitting or slapping. The sense of "to exchange, barter, trade" is first recorded 1590s, possibly from the notion of slapping hands together as a sign of agreement in bargaining (as in strike a bargain). Related: Swapped; swapping. The noun in this sense is attested from 1620s; earlier "a striking, an act of striking" (mid-13c.). Swap-meet attested from 1968, American English. (etymonline)