Learn Russian: Я сам видел эти паруса.

Yes, парус (sail) is masculine. Normally, masculine nouns take -ы or -и in the plural. However, парус belongs to a special group of masculine nouns that form their plural with a stressed -а. Other common examples from your current lesson include дома (houses) and города (cities).

Since the sails are the direct object, why aren't they in the accusative case?

They are in the accusative case! For inanimate objects (things that are not alive, like sails), the accusative plural form is exactly the same as the nominative plural form. Since the nominative plural is паруса, the accusative plural is also паруса.

Why do we use эти instead of этих for the word these?

Because эти modifies паруса, which is inanimate. When an inanimate noun is the direct object, any adjectives or pronouns attached to it also use their nominative plural forms. If you were talking about living things like animals or people, you would use the accusative animate form этих.

Why use сам for myself instead of себя?