My appologies, that should have been Traveller.

Where I didn't look before, Gmelch, p. 23: The use of Shelta as
the language of the road also strengthened their [Travellers]
identity. As used by travellers today it is a true "cant" or form
of disguised communication, whose purpose is to conceal the
meaning from outsiders.

Note that the Traveller horse seller (blocker) in that bid setup I
described above must be a master of applied psychology.
Having haggled to some price, they go through another cycle.
'blocker' finds some new perfection in the horse and talks it up,
watching non-Traveler's facial expression and any of his body
language for clues to his thinking. 'blocker', judging when he
might get another pound higher bid, uses Shelta to signal
Traveler dummy-bidder to up the bid by a pound. Non-Traveler sees
dummy bidder knows his horseflesh so its a good deal, and worth
it to up the bid a few more shillings to get such a magnificent
animal, while 'blocker' is studying the horse for new
perfections, and so repeat the cycle.