Mark Twain related the following true story in his autobiographical book "Life
on the Mississippi" in the 19th Century (a "drummer" meant a salesman in those
days):

Speaking of manufactures reminds me of a talk upon that topic
which I heard--which I overheard--on board the Cincinnati boat.
I awoke out of a fretted sleep, with a dull confusion of voices in my ears.
I listened--two men were talking; subject, apparently, the great inundation.
I looked out through the open transom.  The two men were eating
a late breakfast; sitting opposite each other; nobody else around.
They closed up the inundation with a few words--having used it,
evidently, as a mere ice-breaker and acquaintanceship-breeder--
then they dropped into business.  It soon transpired that they
were drummers--one belonging in Cincinnati, the other in New Orleans.
Brisk men, energetic of movement and speech; the dollar their god,
how to get it their religion.

'Now as to this article,' said Cincinnati, slashing into the ostensible
butter and holding forward a slab of it on his knife-blade,
'it's from our house; look at it--smell of it--taste it.
Put any test on it you want to.  Take your own time--no hurry--
make it thorough.  There now--what do you say? butter, ain't it.
Not by a thundering sight--it's oleomargarine!  Yes, sir, that's what
it is--oleomargarine.  You can't tell it from butter; by George,
an EXPERT can't. It's from our house.  We supply most of the boats
in the West; there's hardly a pound of butter on one of them.
We are crawling right along--JUMPING right along is the word.
We are going to have that entire trade.  Yes, and the hotel trade, too.
You are going to see the day, pretty soon, when you can't find
an ounce of butter to bless yourself with, in any hotel in
the Mississippi and Ohio Valleys, outside of the biggest cities.
Why, we are turning out oleomargarine NOW by the thousands of tons.
And we can sell it so dirt-cheap that the whole country has
GOT to take it--can't get around it you see.  Butter don't
stand any show--there ain't any chance for competition.
Butter's had its DAY--and from this out, butter goes to the wall.
There's more money in oleomargarine than--why, you can't
imagine the business we do.  I've stopped in every town from
Cincinnati to Natchez; and I've sent home big orders from every
one of them.'

And so-forth and so-on, for ten minutes longer, in the same fervid strain.
Then New Orleans piped up and said--

Yes, it's a first-rate imitation, that's a certainty;
but it ain't the only one around that's first-rate. For instance,
they make olive-oil out of cotton-seed oil, nowadays, so that you
can't tell them apart.'

'Yes, that's so,' responded Cincinnati, 'and it was a tip-top
business for a while.  They sent it over and brought it back from
France and Italy, with the United States custom-house mark on it
to indorse it for genuine, and there was no end of cash in it;
but France and Italy broke up the game--of course they naturally would.
Cracked on such a rattling impost that cotton-seed olive-oil couldn't
stand the raise; had to hang up and quit.'

'Oh, it DID, did it?  You wait here a minute.'

Goes to his state-room, brings back a couple of long bottles,
and takes out the corks--says:

'There now, smell them, taste them, examine the bottles, inspect the labels.
One of 'm's from Europe, the other's never been out of this country.
One's European olive-oil, the other's American cotton-seed olive-oil.
Tell 'm apart?  'Course you can't. Nobody can.  People that want to,
can go to the expense and trouble of shipping their oils to Europe and back--
it's their privilege; but our firm knows a trick worth six of that.
We turn out the whole thing--clean from the word go--in our factory
in New Orleans:  labels, bottles, oil, everything.  Well, no, not labels:
been buying them abroad--get them dirt-cheap there.  You see,
there's just one little wee speck, essence, or whatever it is,
in a gallon of cotton-seed oil, that give it a smell, or a flavor,
or something--get that out, and you're all right--perfectly easy then
to turn the oil into any kind of oil you want to, and there ain't anybody
that can detect the true from the false.  Well, we know how to get
that one little particle out--and we're the only firm that does.
And we turn out an olive-oil that is just simply perfect--undetectable!
We are doing a ripping trade, too--as I could easily show you by my
order-book for this trip.  Maybe you'll butter everybody's bread pretty soon,
but we'll cotton-seed his salad for him from the Gulf to Canada, and that's
a dead-certain thing.'

Cincinnati glowed and flashed with admiration.
The two scoundrels exchanged business-cards, and rose.
As they left the table, Cincinnati said--

'But you have to have custom-house marks, don't you?
How do you manage that?'

I did not catch the answer.