Look lots, buy little and only stuff you realy want or can shift to someone straight away is my advice as its very easy to get stuff that seamed a good idea at the time and then you find it in a kit bag five years later wondering why you bought it.
Get to know the staff and unless you are a right git they will look out for things you are after. The especially appreciate (this is assuming that you don't want it yourself of course) being told that something is worth a lot more than they are asking. After all they are doing it to make money for good causes so I never feel guilty about depriving someone else of a bargain.
When examining clothes apart from the obvious like great big holes in it a good way of telling how much wear it's actually had is by looking closely at the zip pulls these are usually painted or made of a metal that shows wear. If the edges are chipped its probably seen a lot of use, been washed meny times so you may want to examine it closer before buying.
The amount off fluff in the velcro, although easily combed out (especially if you have dog brush in the house) is another good sign of how much service the garment as seen. It may not matter if you are just going to hammer it at work but if you are after the best you can get for the money its worth being fussy. So many people buy outdoors kit, wear it a few times and it ends up in Help the Aged or wherever you can be choosey.
In the back bedroom is a pile of rucksacks nearly three foot high for the kids and wife to use and maybe two of them came out of proper shops and none of them had been used more than a few times or I wouldn't have bothered with them. At most they needed a wash.
Sorry I'm rambling on but there is so much good kit out there carboots, fleas and charity shops....
Lets see in the last 7 days I found
2 unused pure wool white blankets (ideal for dying for once) not Whitneys but some Manchester company £2.95 each
Unworn (unless someone liked a plastic tag sticking in there neck) Large bright orange Karrimor fleece £3
Karrimore Lynx 3 rucksack, needed a wash, no scratches on buckles) £2
Issue Goretex DPM trousers (some wear on very bottom ) big enough for my fat gut £1.50
5 unused Cutty clay pipes still in the sawdust 50p each
a sack full of reference books etc etc etc, its a rare day I don't find somethng in my dinner hour. Oh the week before last I got a modern Dutch Louet S15 spinning wheel for £20! Ok I refurbed it but only because I was giving it to some one, realy it just needed wiping down.
Ok, I'll get my coat
Tom