Well what do you mean by 'ripping' someone off?

If you are talking about fraud, then sure it is unethical.
If you aren't talking about fraud then you can't 'rip' someone off since trade requires a reverse inequality in wants. That is what people like ClouD don't understand which is why he thinks everyone who makes money is being dishonest in some fashion. A reserve inequality in wants goes something like this:

Let us say person A has a stockpile of peanut butter, a stockpile indicating that they have an overabundance of the peanut butter. Now let us say person A runs into person B. Person B has a stockpile of chocolate, a stockpile indicating that they have an overabundance of chocolate. Person A, desiring a new product or perhaps for some subjective desire, desires chocolate and Person B feels the same. If Person A and/or Person B didn't feel this desire then the trade wouldn't of taken place unless under the threat of coercion but that is another matter. So Person A desires chocolate but has peanut butter and Person B desires peanut butter but has chocolate. Thus the reserve inequality in wants.

Person A: Doesn't want peanut butter, wants chocolate
Person B: Doesn't want chocolate, wants peanut butter

In order for there to be a trade B must have what A wants and A must have what B wants. Therefore, you cannot 'rip' someone off in a non-fraudulent manner.