Two factors play into increasing your product's sales on Amazon:
Traffic & Conversion Rate
Both factors can be heavily influenced by sellers.
Increasing Traffic (sessions) for Products on Amazon can be achieved with:
External Marketing (Social Media, Forums, Influencers, Homepage)
Click-Through-Rate Optimization (Main Images, Title, Prime, Price, Rating, Brand)
Increasing the Conversion Rate for Products on Amazon can be achieved with:
Management of Reviews / Question & Answers
Price Promotions / Stroke Rates / Multiple Units Discounts
Variations (to share reviews)
PS. First and foremost though, you should have a quality, stand-out product.
Find it with proper Product Research!