In message , at 19:26:21 on Sun, 2 Oct 2016,
tim... remarked:
It's not a sustainable business to sell things cheaper than Amazon,
while also paying them to use the platform.

It is if you charge people for postage

What? Less postage than Amazon, who will have negotiated the very
best discounts.

IME (not necessarily with Amazon) postage is charged in fixed chunks
with say 6.95 as the smallest chunk,

That's simply not the case. eBay in particular encourages traders to
ship with zero postage charges. Lots of stuff "fulfilled by Amazon" has
a zero postage charge for the slowest service.

when all the delivery of the specific item requires is a jiffy bag and
a 2nd class stamp

not everyone has Prime

Prime is faster, there's also their free postage offering.

The things that I looked for didn't "qualify"

Not qualifying for Prime is an indication that the product is being sold
on their platform, but fulfilled by the trader. Which is the modality
I'm saying is difficult to make competitive.

I just tried a random example: a pack of four Duracell rechargeable AA
batteries.

You can get them 'direct' from Amazon at £7.44 with free delivery, and
qualifying for Prime.

Just two other vendors match that price, and have free delivery (without
qualifying for Prime).

50 other vendors will also supply, with prices going up from £7.88 to
£8.99 by the tenth cheapest, and to £10 and beyond. The cheapest which
is "fulfilled by Amazon" (and thus also qualifies for Prime) is £7.99,
and rises rapidly to £10.84 for the most expensive (of the five/50 using
this method).
--
Roland Perry