Yes Twinklebells is right - if they request to return for a refund then sellers have no option but to agree. It's much easier to accept the return and protect your account, generally you find that buyers won't neg if you are accommodating most will leave a positive comment regardless or no feedback at all.
Mind you having said that, I recently got a neg which ruined my 100% positive score because what I sold didn't match an item the buyer bought over a month after mine ! The buyer made contact saying that they had a problem, I agreed to a refund upon return or to exchange for a different colour (as I sell the item in lots of variations) but the buyer said they didn't want the hassle of packaging it up so left me a lovely red dot stating that the item didn't match something else they now own so was no use to them
Sometimes on Ebay you find that whatever you offer you just can't win with some folk but it's still better to be seen as a reasonable seller than not.
Twink / 80s - You are both ALMOST correct, but not quite.
If the seller is a private (not business seller), then they are not obliged to accept a return unless the item is faulty, i.e. "buyer remorse" is not a sufficient reason
even if the shoes were clearly labelled / photo'd as a 6, the seller could still open a case and claim something along the lines of "I am a 6 and these do not fit, therefore they are not a 6, the sizing is wrong", and ebay may or may not side with the buyer and force the seller to refund.
it's totally up to the seller if they wish to refund or not at the first hurdle, however, as above, if the buyer decides to involve ebay, they MAY force a refund at the next stage
if negative feedback is left, try live chat, they are usually quite good in removing negs in certain circumstances, and this sounds like one of them.