Should I feel bad?

I saw this lot on E-bay today, and I couldn't resist the $3.84 buy it now price. The item is obviously mis-described as the normal 1340-1 souvenir sheets. Instead, these are the rarer, imperf and numbered issue from the Israel 98 show catalog. These normally go for at least $20. Am I taking advantage of the fact that the seller doesn't know what he has? Or, am I just taking advantage of my knowledge of the stamps to make a shrewd buy? I did consider e-mailing the seller about his mistake, but decided against it. In the past, when I see items at shows that are mislabelled, I have pointed it out to the dealer, and they usually are happy to learn about it. But in that case, I am usually talking to the dealer about other items he has, or I have seen the dealer at the show before. In this case, I don't know the seller at all, and may or may not see items he is selling in the future. What would you have done?