What percent of stuff given to charity shops makes it onto the shelf?

It's difficult to estimate the percentage of donations to charity in the form of consumer goods which make it to the store shelves.

Many donations are simply made as an easy way to get rid of junk; they can't be offered for sale. Even good items are frequently rendered unsaleable by being stripped of useable or attractive parts and trimmings before donation.

Garments often have buttons and even zippers cut from them; this means they cannot be sold in charity stores, since charities simply don't have sufficient staff or volunteers to replace these essential finishes. Even the very best of garments must, sadly, be turned into rags for this reason; there is no alternative.

Of goods which are able to be offered for sale, all make it to the shelves. Staff integrity is, in all reputable and viable charities, rigidly enforced. Workers, usually unpaid, put in long hours cleaning and otherwise making articles presentable for sale. All charities have policies in place to cover unavoidable situations where staff members might wish to purchase an item before it's offered for sale to the public.

You need to contact the particular charity in question to discover their policy in these circumstances; the government agency which oversees your local consumer laws will also have information on these matters.

If your question is prompted by suspicions that a particular charity is not passing on donations for the purpose they're given, try to quietly check on those fears and, if possible, report suspected breaches to the directors of the charity. No reputable charity will allow such problems to go unchecked.