Was having an argument with my flatmates about this. I personally do this all the time and don't see the problem with it. I always think:
1) Clothes in a mainstream highstreet store like Topman have usually been tried on so many times they're basially "used" anyway
2) If I can get, say, an Allsaints bit of knitwear for £10 rather than £80, who cares? I can always sell it on again anyway, and quite often make a profit.
What are you guys thoughts on this?