I appreciate what you're saying Steve, however, the items people buy to sell are highly unlikely to end up in landfill as they are the saleable ones.
The items they purchase are the good quality items that other people would be delighted to buy and wear because they couldn't afford them otherwise. That's the bit that doesn't sit right with me, along with the often extortionate mark ups.
Clothing/textiles are rarely binned by charities as they can sell them in bulk as 'rags' that are recycled. That's going to be the washed out Primarni t shirts and generally ugly clothing they couldn't flog rather than stuff from Next, Joe Brown, Fat Face etc.
I buy a lot of my clothing from charity shops and do look for the 'good' names because I'm on a low wage. There's a lot of us around who need to do that. Maybe that's why I find it particularly annoying.
I know the charity gets their money whoever buys the item. It just pisses me off that the items are bought by profiteers rather than poorer folk who can't afford the price new/marked up on eBay.