I usually buy the T-Shirts outside the venues for the simple fact that I normally get them as souvenirs and not so much as real clothing. I do not run around in band T-Shirts very often these days so the cheapos do it for me. And yes, they are normally worse in quality, especially those that do not come in the basic colour of black or white. The prove is in the laundry if you do not wash the multicoloured ones seperately. So much for the house man's tips.
On the matter of illegality I actually have a different view. I pretty much suspect that those outside cheapos are part of a well thought deal. In the old days you would get the odd guy hidden near the tube station offering you the illegal merchandise. Those folks were always in danger of getting caught or even getting badly thrashed i.e. if they happened to sell non-franchised Guns And Roses stuff. In stark contrast these days the dealers stand right outside the venues in large numbers and appear far from being precautious. Though it would be easy to round them up if anybody wanted to. But nobdy seems to. So probably there is something like a sub-license given out for that sort of merchandise or it is actually part of the whole thing itself in order to catch those cutomers you usually wouldn't. They first sell the stuff to those who are willing to spend the big money for the better quality and the wider range of goods. And the certainty of getting decent stuff. Before, during and after the show. So what do you do with those less merchandise committed fans that have decided against buying? Give them another opportunity and approach them outside the building with the cheap stuff. If they don't buy you haven't lost a penny because they are non-buyers anyway. But those who do (like myself) give you the extra money you otherwise wouldn't have got. Because without the cheapos we would have simply gone home without spending a bit.
This does also explain why we only get the cheapos after the shows and not before as well, though a real illegal dealer obviously should also go for his business before the people have spent their money inside. The true management does want you to get to the expensive stuff first in order to make the bigger bucks. Obviously. And the less you know about the prices and products you may get outside the better.
So it is not a competition between the "legal" inside merchandise and the "illegal" outside stuff - they are in fact complements. The inside offers you high quality and a broad range of goods for a high price. Plus the security to get decent fan stuff in the first place. The outside dealers instead offer the cheapos for those who otherwise would not buy a thing anyway. And those people (like myself) seem to outnumber those who would have bought expensive stuff if there wasn't a far cheaper alternative. Inside, outside - two different channels for two different kinds of customers.
And I want to believe
In the madness that calls 'now'