I think there's a difference between getting a deal and screwing somebody over.
Somebody selling a Turbo Duo at a flea market for $8? That's a good deal. I don't think the buyer has any moral obligation to say anything. If he does anyway then that's just going above the standard making him a better man that I am. Now saying something after the purchase? "Yo, this Turbo Duo you sold me for $8? Ha, it's worth triple digits." That's just being a dick. Say thank you and leave.
Now if you bought the Turbo Duo for $8 and there's a CD in there the seller didn't know about? Or the seller made a computation error and gave too much change? I think there's a moral imperative to at least make a reasonable effort to bring up the issue. In some cases this might be impossible. You buy the thing in Florida at a random yard sale while on vacation and don't realize something is wrong until you get home to California. But in most cases I think honest mistakes of that sort are relatively easy to spot and easy to fix.
The reason I see it this way is because I'm a big balance junkie. If the reverse happened, where the seller made a mistake and sold the unit as less than he intended the buyer would rightfully expect the seller to do something about it. If the Turbo Duo were sold as working for a normal price but it turned out to be broken the buyer would expect either refund of all or some of the money or for the seller to fix the item. If the buyer is forced to eat a bad deal it's often done begrudgingly so.
Where is the balance if the buyer says "if something goes wrong for the seller and right for me then I win" and simultaneously "if something goes right for the seller and wrong for me then I refuse to lose"?