Honestly, it depends upon what was offered. Classified cover such a broad range that any blanket statements would likely encounter a plethora of situations in which they were inappropriate. I do believe that it is the seller's responsibility to declare an item "as-is, no refunds" if they want the buyer to assume the risk, but that seller should also expect to get much less money for items sold as such, since the seller has shunted the associated costs of that risk.
If the classified was listed as "as-is" *and* the description was accurate, then the buyer has no reason to complain once the transaction is complete. Example, seller lists, "XT Arabic keyboard, no way to test, unknown condition, sold as-is" and upon receiving it the buyer requests a refund "because there's no way to plug it into an iPad", then yeah, that's the buyer's fault since the seller never said it would. If, on the other hand, the buyer receives it and the back of the case is one big melted scorch mark, then the condition was misrepresented and a refund would be appropriate.
On the other hand, if the unit was not listed as-is, and the unit fails under normal use within a reasonable amount of time (30 days of purchase seems usual), I don't think requesting a refund is out of line, as the seller sold defective goods -- presumably unknowningly, I'm assuming no way to demonstrate malice from either party. Does that suck for the seller? Sure, but that's also part of the cost of doing business, and should be worked into a seller's prices; it's stupid to bet against entropy, something will always fail. The seller would also be responsible for shipping costs, transaction fees, etc, since again, they sold defective goods, and these costs should be worked into prices (or accepted as a gamble on the seller's part).
As far as a buyer "finding a better deal" -- that's a tougher needle to thread. If the item was not sold as-is and the buyer returns the item in the same condition, again, within a reasonable amount of time, then there's no reason for the seller to refuse a refund. The buyer would be responsible for getting the unit back to the seller (shipping costs, insurance, etc), but the seller would be responsible for any money transaction costs (PayPal fees, credit card percentage, the stamp to mail back the envelope full of cash, whatever), since it was the buyer's decision to rescind the transaction, but the seller's choice for how to accept funding.
On the whole though, the onus will always fall more heavily on the seller, since a reputation for being unreliable in any way hurts a seller far more than a buyer.
