Should I feel guilty?

[Lady-Loki]

You should not feel guilt. The responsibilty is on the seller to verify what he puts in his shop. If he does not check & double-check then he is the only one guilty in this case.

Had you not bought the item someone else most certainly would have, so the seller would still be without that item and would still have exactly the amount of coins he set his shop to sell for.

You do not need to feel guilty since you did nothing to cause that item to be sold at the price he set his shop for.

From FTC Website:

Because overcharges can occur in all stores and on all kinds of items, the report recommends that consumers carefully compare advertised or posted prices to scanned prices at checkout. Ultimately, however, responsibility for accurate pricing rests with the individual store and its employees. The organizations participating in the study urge retailers to continue to examine and, if necessary, change their pricing practices voluntarily.

The above is addressing overpricing but would apply to underpricing also. The key point in the above is the bolded & underlined portion.