Oxfam overcharging for what they perceive to be rare items (and it pretty much is Oxfam we are talking about here) is a matter of fact ... whether it's actually worth getting upset about is another matter.
On the stuff they don't perceive to be rare items - basically anything not in RCRRPG (what's that these days? anything worth less than a tenner if minty or has it gone up?) they charge less than a typical record fair - £3 or so usually in our nearest. While this is more than the other local charity shops charging £1 to £1.50 they do at least do us the favour of only stocking decent stuff. ie. you don't have to shift through ten copies of
Let's have a Max Bygrave Hammond Organ Party for every potential purchase.
I would argue they are fairly good value for certain sorts of records (anything worth less than the RCRRPG minimum). Anyone who bought something rare and expensive from Oxfam would need to be pretty sure of what they are buying and get lucky. Can't say I've ever been tempted, but I don't go into a charity shop in an expecting to drop £20 or £30 on a record frame of mind
I suspect a lot of charity shops have dealers getting first dibs as it's the only thing that would explain that they only ever seem to have rubbish. The best locally to me for lucky finds is The PDSA, which seems to have a policy of putting out whatever comes in, which means if you get there at the right time and hit a good collection you can make quite a haul. Only happens once in a while though and you need to visit regularly and early.
I get the impression that's quite popular these days, often a band will release a single as a download and a highly limited and desirable 7" single, some even signed or with hand-made sleeves etc. There are some really nice items out there, I've been known to buy randomly just because I like the sleeve! Some can be good investments too.
They can be, especially in the DIY record label scenes, where runs of singles are often as little as 250 copies. I've got quite a few indiepop singles released in the past couple of years which are now worth £30 or £40 (at least if ebay sales are anything to go by) and at least one worth over £100*. No intention of selling them mind you and no knowing of those prices will sustain as the prices just reflect a rare item desirable to a band's usually relatively small (but larger than the run of singles) fanbase.
* Pains of Being Pure At Heart/Parallelograms split on Atomic Beat Records. Purchased because I know various members of the Parallelograms and the lady who runs Atomic Beat from her bedroom quite well ... oh and because it's a good record
. All of whom are quick to point out that the record isn't worth this much because of anything they did, but simply because the Pains are having a reasonable amount of success and have an obsessive US fan base who want rarities like this badly.