Yes, the definition of a best-selling album is one that sells the most, and makes the most profit. Everyone has their own opinions on music, so the term "best-selling" should be determined through profit alone. The best-selling album of a year is the album that sold the most, not necessarily the album that was most highly-regarded.
I am sure artists would like to have the most profit. That all depends what kind of deal they worked out with their record label though. Since all of that is different from one artist to the next, they have to go by unit sales. Most units sold, equals the best selling They can put the most profitable artists in a different category.
Some albums are overpriced, and others go on sale really soon because they are so bad. But we cannot say that the best-selling ones are the ones with the most profit because more people could have bought the special edition of a CD even though the competitor's do not have such an edition.
I do not believe that profit alone is what should determine if an album should be considered a best seller or not. Sometimes, the profit margins of an album not that great due to costs of the artists and production when making an album. I think that if an album sells a good amount of copies, it should be considered a best seller.
No, the spectrum of music is able to be shared through various sources on the Internet. Although one would like to believe that music is bought legally, there are tons of ways to get music for free. Many albums are obtained that are not counted in the profit section, meaning the profit of any album is misleading.