This is something I wonder about in connection with remote places like e.g. Alderney. People there struggle getting stuff from Amazon, for example.
You can cost share with passengers but even bringing a box of matches for a fee is illegal. You need an AOC for cargo flights.
However, if you bought the box of matches, for say €1, flew it, and sold it for €1.01, then you are running a business, and a PPL holder can fly in connection with his business, so this is legal. Even if you sold it for €0.99 it is still a business; there is no legal requirement for a trading activity to be profitable.
Whether the tax authority actually classifies you as a business for tax purposes is up to them. This is the case under all “modern” jurisdictions, I am sure; it is done to avoid the obvious ploy of offsetting depreciation on personal effects (most of which are almost worthless the day after you buy them) from your tax liability 
But that is probably not relevant to the paragraph above, where reselling the box of matches makes you a “trader” and makes it legal.
Peter wrote:
However, if you bought the box of matches, for say €1, flew it, and sold it for €1.01, then you are running a business, and a PPL holder can fly in connection with his business, so this is legal
So if my business is shipping matches, and if I use my plane to ship them, this is legal?
The items carried must be the property of the aircraft owner. That also means they cannot be prepaid by the customer. But they can be ordered by the customer.
How he obtained them (made them, bought them, etc) is irrelevant.
What I don’t know is whether there are different local regulations (e.g. not allowed to carry goods for re-sale) around Europe in this area. @bookworm might know but we haven’t seen him for a long time.
Matches is perhaps a bad example because 200kg of matches might be “dangerous goods” 
I am informed that one supermarket on the Channel Islands bought an Islander (or similar) and used it legally for picking up stuff for their shops. They didn’t need an AOC for this. An AOC was later obtained for carrying paying passengers (obviously).
